Knee jerk answer is the 64 bit version as it would be able to use >> 4gb if this system was replaced in the future. (Athlons... DDR3... etc)

Or go Linux 64 and run Windows virtually so it almost doesn't matter what the host hardware is. This might bypass that pesky activation nonsense should the host hardware change dramatically in the future.
32 bit maxes out somewhere between 3 and 4gb (at least for XP).
Also make sure you get the version of seven that includes a free virtual pc of XP. Especially if you do not have any licenses left. I think Business Pro is the lowest that supports/includes it.
If you have hardware that can support it the only downsides to 64 bit is the loss of legacy support and/or some programs may not run quite right on it. I've been hemming and hawing about putting 64 bit SUSE, 64 bit XP, or 64 bit Windows 7 on my laptop for a while now. I think I want to get a slightly better model that maxes out at 8GB of RAM and has a sata based ultrabay instead of pata.
My laptop is perfectly suited for what I currently use it for. My desire for something marginally better annoys me but buying peripherals that would
only work on this model bugs me too. (I need a new battery already).
DarkHumour