Having too much ram is not that needed, 4GB is totaly fine, what matters is a good CPU and a VideoCard.
Also i prefer 32bit (x86) systems (at least as a programmer/reverser) because i never experience ANY problems with ANY software (well nearby 99% of the soft works flawlessly on x86 W7).
The nonpaged memory from the ram works flawlessly, as you are using 3.25 and the rest is "for extreme cases". If you setup more ram (which is NOT NEEDED) you would like to upgrade to x64 and feel problems one after another (at least this is how was for me).
@dschallert, i think you can still stick to your current configuration and freely upgrade to W7 without worries (and you decide on 32 or 64bit, but I advice for 32). Wait another 2-3 years and everything may become 2x cheaper, then you may get an alienware PC?

Also if you plan on using lots of games and such, and you are running nVidia or ATI videocard, you may experience great optimisation at framerates in many DirectX games because of the well-coded style in W7, which was not intented in XP at those times. Here is an example of tests for FPS in W7 and XP on the exact same config:
http://www.youtube.com/watch?v=7TPifL_9IFM