New PC vs. old photoshop

Well, since asking for advice about upgrading to a new PC to upgrade from my old P4, I've spent the last week researching and following up ...and learned some facts I'd overlooked....1st and foremost, when I'd asked for advice I hadn't mentioned which PS version I was using...upgrading PS isn't in the cards for now so that was a critical factor...
1. Older stand-alone Photoshop versions and CS1/2 will not work on VISTA OS...
Well, CS2 does, with some issues according to Adobe...
2. Older Photoshop versions would not benefit from quad core CPUs...Core 2 Duo CPUs seem to be the best match for them...
This limits older PS users to WinXP OS...something I'd not considered...it does limit my available options...
As for CPU options...the E5200 with 3mb RAM seems to be the sweet spot for $/performance...as XP doesn't access more than 3mb RAM, but I gather 'faster' rated RAM will help...
Thanks for your help and advice....I've learned a lot from you all....

XP can address up to 4GB as it is a 32bit OS. That means 'total' memory so minus your video card RAM from 4GB + some added address space for hardware. With a 256MB Video card XP will see 3.5GB with a 512MB video card your OS will see 3.2GB and so on. Memory is cheap! Get all you can. In any case to achive 3GB one of the modules would have to be of a different size, which is not recommended and it will likely cause dual channel memory mode to go by-by which will slow you down. Best to get 2x2GB sticks or 1x4GB stick.

And I wouldn't shoot for the sweet spot - I would think about future compatibility and speed. I cannot remember how older PS runs on Vista, but I do remember it working with some compatibility tweaking. As far as CPUs, always try to get as many as you can. Even though PS is not a heavy out-of-order software package the OS will try to distribute tasks across all CPUs to increase bandwidth. The OS can send tasks to other CPUs that are not peaking out thus increasing PS response time.

Get quad core, even if PS doesn't use all four cores right now you'll upgrade one day and it will. I suspect it will use the cores more fully than you expect, I can't check as I uninstalled CS2 last week.