I almost always use the RoMsmartmap script. I don't remember the size of the map exactly, but it's 5-6 steps higher than the last named setup (huge? gigantic? ginormous?). I usually do it with 75-90% ocean, so that navies are somewhat important. Don't really run into much slowdown until the industrial age. I'm sure the future era is cool, but the game is usually over by then.
On my old PC, 32bit XP, can't remember the video card, but it was an AGP card, 4GB RAM...I would have to keep the maps in the large area, and it would still slow down significantly towards the end of the middle ages. A huge map in vanilla would slow down quite a bit.
My new PC is a Phenom II 955, 8GB RAM w/Win7 x64, and 2xATI 4870X2's. Dual screen setup, running in windowed mode. Usually with a few tabs open in Firefox, a couple IM windows, and occasionally an Eve client or two running. It's awesome.