A 64Mb GF4MX isn't a BAD option, most effects are supported and its nice and quick, should set you back around £60, but if you can afford it (at the present I can't

) a Ti card is a better budget option. If money is not a factor, the latest cards not quite released are promising, looking at specs in a few PCMags recently, the biggest GeForceFX outperforming anything from ATI, but only Juuuust. Plus a friend of mine has one and it p*sses me off by being stupidly noisy.
I would personally say WinXP if you are buying a brand new system, or 98SE if you are upgrading an old one. On a system with components desinged for XP I have found it very stable. The Home version will definitely suffice unless you want to network it. Win98SE is probably the most stable Windows for older pcs, I've had no end of trouble with the various NTs, ME and 2000.
Processor wise, AMD is actually far ahead in the majority of performance issues to P4. They are far better value for what you can get for the same price. I would personally recommend an AMD 1800XP as these currently offer outstanding value for money

if you can afford one, plump for a 2300XP or higher, and again if money is no object, look into dual processors, you can easily push 4Ghz that way
Obviously go for the fastest components you can afford, hdd, ram etc. And with RAM obviously more is most definitely preferable to less.
I recently bought parts and made up a AMD1800 machine with 1Gb Ram, GF4MX and 120Gb HDD for just under £350 which I think was a bargain. I've had virtually no problems with DBPro (Patch 4) programs slowing down, even when I reeeeaaallly push them
Obviously if you have more money buy more hardware, but the above spec I have more than suffices for myself. Heck I got on pretty well with my 800Mhz GF2 system
Two areas really worth paying more to improve for gaming are A: Monitor (pref flat screen, 19"+ I have 21"

) and B: Sound (5.1 or 6.1 surround sound is awesome if set up right

)