This was the 4th space victory I have done for Civ4 and 3 of them are BOTM29, SGOTM12, and BOTM37. I'd like to share with you my thought of key factors for a space victory and hope that you could gain some useful ideas, although the results were not optimal due to my suboptimal play and greedy for Gold Medals.
I BC Era
1. REXing -- Land is power, this applies to every difficulty level, including deity, although in higher levels, the barbarians, AIs, and maintenance are going to pose more challenge to do so. Tips -- settle your 2nd city as fast as possible and usually try to stay close to your capital (because your 2nd city will usually boost both net commerce and production), except that the barbarians stops you to do so, which in my experience, only in deity level, you probably need a stronger unit or several warriors for fogbusting.
This is my Rexing result in 5BC, 11 cities with 310 BPT.
How to achieve this in a map with low production, many of you have already known the answer -- whipping, however, many of you have not been aware of how powerful of whipping, This is how I whipped my capital, Washington had been constantly whipped of a worker or a settler at size 4~6 and the overflow went to wonders and units, this was how the hammer poor capital did the job of REXing and wonder spam.
2. Wonders -- For a map that you are able to settle more than 5 good coastal cities, the most important early wonders are GLH and The Colossus, financial leaders fit best with these 2 wonders. The 2 wonders enable you to REXing until the land runs out. Oracle is also a good wonder to pursue, but IMO is not that great as the above 2. Many people are fond of Mids, which IMO is only a mediocre wonder and I have never build it without Stone. My reason is that HR is better when you are facing happiness issue (i.e. massive whipping and/or not enough happiness resources). On the other hand, representation is a more expensive civic and in early stage, most of the time you want slavery and therefore you won't run many specialists to take the benefit of Representation.
As you can see from the last screenshot, my capital got Oracle, GLH, and TC and was building TGL.
3. Techs for REXing -- GLH and TC allows you to REXing in early stage since almost every coastal city pays for itself or even boost your net commerce at the time it is founded, however, to REXing non-stopping, you need more -- Currency, CoL are the 2 key techs. My early teching is like BW...Oracle techs and Oracle took MC since Roosevelt is Industrious and Forge will help on both production and happiness ... CoL->Math->CS, which enable me of expanding like no tomorrow.
4. Great Person -- In most of the game I have played, my major GP farm is capital and the GPP come from wonders but not specialists. I usually try to grab TGL and immediately followed by NE in capital, this 2 wonders alone already contribute 18 GPPs or 27 GPPs if you play with a Phi leader. There's little problem to produce ~3 GPs before liberalism date, which is enough even for a deity game. BTW, many people usually don't care about ToA, which IMO is a very good wonder (better than Mids) when have access to Marble and could build it early enough.
II AD Era
1. Infrastructures -- The peaceful REXing usually ends now, time to whip/build the infrastructures. I rarely lay down cottages since I rate infrastructures much higher than cottages and farms enable you to whip infrastructures without any hesitation and I'm really very cruel at whipping. Because capital was low of hammers, so I had never stopped whipping the precious capital, how to solve the happiness issue, easy, HR plus a bunch of warriors.
2. Techs and wonders to boost your BPT -- Literature for TGL and NE, mainly to setup your GP farm. Music for the free GA. Edu for OU. As you have seen from my 5BC screenshot, I was only 5 turns from completion of Edu, however, it's still a long time to have OU online since most of my cities were small because they were fully contributed to REXing. In my mind, getting good cities founded as early as possible is more important than saving pops for faster OU. As a result, what I did after Edu, simple, I set slider to 0% until OU was completed, this is why you have seen over 8K cash in the following screenshot. So in 580AD, I have settled to the island cities nearby and to the other continents as well. BTW, I got 2 techs (Calendar and Aes from the Christmas presents).
3. Expanding with war -- As you have expected, war is the only effective way to keep growing after you have run out space for peaceful expansion. I waited until I got cannons for cheap war, the 1st target is obviously Japanese due to the massive gold/Gem/Silver mines as you can see that I have already have AL and RR and Mine Corporation available.
This was how I whipped my Shine city and Corporation headquarter. Corporations are definitely the optimal way to play to the fastest space victory, the only downside is that they add much more micros to play the game, the spy assignment by the city governor is extremely annoying!
The 2nd war was geared toward Egypt, they had pretty good land, the most important reason was that they grabbed a bunch of wonders, including the absolutely needed wonder of MoM. BTW, the only GA I launched before I capture Egyptian capital was the one from Taj and I have not used any GP for light bulb.
The war have not stopped, my 3rd target was English and this time I have better forces -- 10 bombers.
And here's the 4th war with Aztec and I have to stopped and liberate a bunch of captured cities since I have taken more land than domination limitation.
Reached the milestone of 10K BPT in 1500AD
1 turn before victory, note that Russian was also down because I was greedy for score milking.
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But reading their posts will at least help us become better.
. Barb's luck had something to do with that.
. The WB that made such a horrible discovery was ordered to suicide against a Barb Galley as a punishment. 
. I start selling techs for money and will do it till the end of the game, always keeping my slider at 100%.
. Use my unlucky-that-got-to-late-to-found-a-religion GS for a GoldenAge and revolt to: Repr+FS+CS+Pacif.
