SImply no need for over the top RAM unless you're planning on overclocking your CPU or some such. I would recommend going with a reputable manufacturer, such as Crucial or Kensington, but you don't need to get their expensive stuff. I went in this instance for PC2-4200 RAM, so 533mhz (on a P4 3.2ghz, Asus P5WD2 Premium) which was £233 for 8Gb - they would have gladly sold me the higher end stuff for £400 or so, but there's just no point.
The RAM is dull and boring and looks like this - not a heatsink or LED or any other rubbish in sight:
In terms of platform, XP x64 has been around for four years now, so is fairly well proven, though not many manufacturers have bothered writing drivers for it, as they worked on Vista instead. Vista itself is very new, and the industry as a whole has seemed to be very slow on the uptake - only last week were Macs finally 64-bit, and despite PCs having 64-bit OS options for so long, very little has been done about it. So if you're going down this route, logic would dictate that XP x64 is the safer bet for now.
However, as I said before, do consider the amount of Cubase/Nuendo features that DON'T work under ANY 64-bit operating system - maybe you could live without native versions of some of the plugins, but the lack of a 64-bit version of Quicktime is out of Steinberg's hands - I guess Apple will roll it out as and when they can be bothered - as mentioned, they have only just made their OS 64-bit, so QT may yet take a while as they fiddle with more phones or iProds or something fatuous.
I currently have a working 64-bit test system (dual boot on my main DAW PC) which is running Vista 64, and seems to be "relatively" stable - however, with no Powercore drivers, no QT, and several of the main plugins and features either limited or disabled, it's too early for me to risk paid work by moving over to it - hence I run Nuendo 4 on XP32, which I know works, is solid, and hasn't fallen over in years.
The slave PCs for Vienna Ensemble however are another matter - because of the newness of Vista, I chose to stick with XP64 on those, and bless the VSL team, VE has worked on both machines without a hitch - but then they're not being asked to do very much - merely boot, load the RME drivers, MIDI-over-LAN and VE. For that purpose they have performed flawlessly.
One final thought - if you find your budget won't stretch to 8gb and you can only get 4, then get 2x 2Gb sticks - it'll be a little more money than 4x1gb, but when you have more cash, you can just slap in another pair of sticks - if you get 4x1gb, then when you come to upgrade they will all be redundant, and will sit on a shelf gathering dust!