I have a Giga-Byte GA-P35C-DS3R Motherboard which can take both ddr2 & ddr3 rams. I was wondering is there a major different between them? At the moment, I'm using 2GBs of PC2-6400 (400MHz) with a Q6600 @2.4 GHz under windows 7 Ultimate. I used my pc mainly for photoshop, illustrate, autocad, rhino, grasshopper and a little bit of gaming. What would you guys recommend me to make my computer faster? A new motherboard with 8GBs of DDR3 rams? or new ssd... etc
For RAM's its most about the amount that goes in rather than what goes into your PC. If you can find a exact matching DDR2 RAM of 2GB, get that. It would be better. Get a DDR3 when you finally get a better system!
If your 2GB is a single stick, then as hell_storm2004 suggested, get a second 2GB stick for 4GB total. If your RAM is 2x1GB, then if you want to add RAM, bite the bullet and switch to DDR3, as you'd be able to re-use it in your next rig.
At what tasks would you like your system to be faster? You don't mention your GPU, but that might be a place for improvement as well. Your CPU should be able to overclock pretty well, 3.2GHz should be easy, and 3.6GHz is also likely within reach.