2GB RAM chips? Taking system to 8 Gigs?

most motherboards support 8 or 16 gb of ram
http://www.newegg.com/Product/Prod [...] 6820231120
you could get two of those sets I guess, but I'm not sure whether you would want to wait for DDR3

Most current mainstream boards w/4 DIMM slots should handle 8GB. However, 2GB DIMMs are VERY new, as is VISTA. Also, Photoshop is currently a 32-bit app.

OTOH, Core2Duo processors available now provide a substantial performance increase over previous CPUs in Photoshop, especially when stably overclocked to boost the FSB to 333-400MHz (1333-1600MHz data rate, requiring DDR2-667 or DDR2-800 memory speed to keep up), which seems to be well within the average C2D's capabilities.

Thus, in your case I would buy a solid mid-to-high end overclocking-friendly MB ($200-250), an E6400 CPU (about $230), 2x1GB of good quality DDR2-800 CL4 RAM, 2x512MB of good quality DDR2-800 CL4 RAM, etc.
That gives you 3GB total RAM now; later, when 2GB sticks are more common (and cheaper!) and when Photoshop can take advantage of it, replace the 2x512MB pair with 2x2GB sticks, for a system total of 6GB.

Here is Adobe's knowledgebase article on Photoshop and memory limits/usage:
http://www.adobe.com/go/320005

Nope, no problem. I'd try to get the same brand/line for both.

In order to run in dual-channel mode, it needs to be increments of two (one in each channel). Modern CPUs like the C2Ds pretty much require dual-channel mode to keep up with their FSB speeds. For example, running in single-channel mode, it would take DDR2-1066 RAM to keep up with a non-OC'd C2D, but only DDR2-533 RAM running in dual-channel mode.