I used to own a computer store. What applications are you trying to run and I can give you a better idea of the type of setup you'll need.
What kind of tests are you running? Are they in a program that takes advantage of multiple cores? Would multiple dual or quad core processors make any sense for the application? Can you afford to wait until Intel and AMD come out with chips that are better than quad core? I think the best fit for you might come down to those questions...
Win XP 64 looks to be an incompatibility nightmare - not just drivers, dll's also. http://en.wikipedia.org/wiki/Windows_XP_Professional_x64_Edition
You might have a bit of a wait. Faster quads will no doubt be out soon, but it seems unlikely that you will be seeing more cores on a chip from intel or amd for a least a year - probably more. Having said that Sun have just announced that they have Solaris running on a 16 core chip. But these are probably somewhat simpler than Intel or AMD because they are not carrying around all x86 baggage.
Suppose you want to backtest portfolio based systems over say 5 years of one minute data over all the components of the SP500. You not only have the OHLCV series to contend with but any number of other time series such as indicators moving averages etc etc. You really do want to hold it in memory, especially if you want to run an optimizer over it. The situation is much much worse for tick data.
I still don't see the need for over 4GB under that circumstance, he's not running a rendering farm or compliling commercial software.
I'm sure if you had experience with backtesting data, you surely understand the need for more power. In fact like one said if you are running tests on data 5 years using 1 min bars, say you have 10,000 tests. I'm willing to bet that will take say 10 hours or so on my 2 GB computer. Also, I looking at this genetic optimizer (a solution rather than buying a more powerful computer). Anyway i'm reading the operating manual and they said with even the most powerful computer being a "cray" (don't know what this it) it would take in the neighboorhood of 1 trillion years to complete....WOW!