Kingston's a good brand. So is Corsair, GSKILL, and Crucial. I think GKSILL and Corsair are the most popular, though. But in a Best Buy or something (I dunno where you'd shop for PC parts, I buy online) you'd probably find mostly Kingston, I think. Which isn't bad but more options is best and it'll probably be over-priced from places like that. As to whether DDR3 and DDR2 is compatible, I don't know. I do know that for the RAM itself, you'd be best off getting nearly identical RAM to the RAM you currently have (especially latency, I think). Although if you're swapping all of it out, then that wouldn't matter so much as the motherboard slots.