there are almost no games that support a quad cpu, imo it is wasted money.
civ is not so very dependant on graphics, all middle class cards are more then fine.
i run vista and have no more problems with ffh2 compared to xp. i like some of its small improvements, no suggestion from me here.
any more in depth discussion is even more off topic here, i would suggest you take a look at pages like anandtech.com or tomshardware.com etc, most also have forums where a whole bunch of hardware-disciples follow their religion.
There are some minor issues with Vista and FFH/FF. Mostly related to multilayer games.
As for games supporting quad core. Yeah... A number do now. But they're mostly RTS and high end FPS.
As for graphics. It all depends IMO on how much you are aiming to spend. An 8800GT/9800GT isn't a huge investment(~100$ here in the usa), and is a VERY nice card. As is the HD4830. But a 9600GT or HD4650/70 is more than sufficient. Stay away from Geforce 9500's they are low end, not mid. Just in a subtle disguise...
That said, even a 8600GTS/HD3670 would be a fine card. Although for FFH/FF I STRONGLY urge it be a 512MB card. Lest you chance issues with large/huge maps.
Tomshardware which was mentioned above, is a good place for comparing the performance of these cards---although note their benchmarks tend to be a little ways off real world, they are correct in relive with each other, so you can use them to compare hardware. Just don't expect your numbers to match theirs.
If you want to go vista. Go vista all the way. X64. 4GB of ram(Or more, if you feel like it). And you wont regret it. It's a VERY nice OS on systems that can completely shrug off its heightened demand for resources. And so far as I've seen, is less buggy than XP64.
If you don't feel like going x64, I suggest XP. Yes, not all of the 4GB of ram you'll likely slap in the system will be put to use... But heck. DDR2 is dirt cheap nowadays.
Cpu nowadays... Stick to a recent dualcore thats 2.4ghz or faster(So no old pentium-D's or old 90nm Ath x2's which while capible, run a bit hot...) and you're unlikely to have a problem. Currently Core2duo's go faster, but at your price mark, its mostly transparent.
Lastly, get a solid motherboard and psu. I made the mistake of going with a semi-budget third-party board. And while its a very nice board... There are a few issues with it. (Freaking sound card... and utter lack of tech support)
Asus, Gigabyte, MSI, Biostar, XFX, EVGA and such make great boards.
Stay away from jetway/foxconn/pcchips.
I built my system this last spring, for 700$ USD. And thats after ~150$ in iritating extra costs.
Specs
Ath x2 5000+ BE (2.6ghz stock, got it running 2.8ghz generally)
(NOTE!!!: Black editions, even retail box, do NOT come with a heatsink/fan, other retail boxes do)
Galaxy 8800GT 512MB GDDR3
ECS ATI 770 based motherboard. (Good board... crappy drivers. Horrible tech support.)
4GB Corsair XMS2 800mhz DDR2 (4x1GB modules. Caught it on sale, would a gotten 2x2GB if it had been on sale at the time)
2x 500GB hdd (One segate, one western digital...)
500W thermaltake PSU
Some DVDRW or other...
Hopefully I didn't overload ya XD... I'm a bit of a hardware geek... so I geek out on it a bit XD...