Jaden, ReadyBoost is designed namely for lower-performance PC's. By adding more RB when you have 2GB + of ram, it does less and less. However, it does help extend the life of your battery if you're on a laptop.
I've got a pretty nice laptop, and I do use ReadyBoost. I can't say I notice a difference with 3GB ram, but I do think it helps increase the battery life. I've used 512MB, 2GB, and 4GB of ReadyBoost flash memory, and I can't say I've seen a difference in performance with any of them.
And in regard to DX10 vs DX9 (which I forgot to address earlier) I don't think there's a huge difference. Then again, I don't know if I've played any DX9 games, but the DX10 games I've played are amazing. Crysis runs great (even at high detail w/ everything enabled), same with TimeShift, BioShock, and COD4.
To say that your computer isn't running well with DX10 is strange, considering the specs. I've only got a 2.2Ghz core2 duo processor, so you'd think a desktop with more power (and probably a better cooling system

) would run better. I'm guessing that somewhere a setting isn't at peak performance..