Help upgrading my rig

Hey guys! I've been thinking of upgrading my PC for this christmas but I need some advice regarding some issues.



My current system rig:



Intel Core 2 Duo 6600 2.4Ghz

Asus P5B Deluxe

4x1Gb 667 RAM Kingston

Nvidia 9800GTX

Creative Sound Blaster Fatal1ty

LG W2452V 24" Monitor

Windows XP



I've been thinking about changing to a quad-core system, such as Q6600, Q8200 or Q9400 but I'm not really sure if it makes such a big difference, or maybe I should get a new core 2 duo such as the T7500? Then depending on the micro I'll need to get a proper motherboard, any good one you would recommend? I'm not really going for the best of the best since I'm not planning to spend too much money, I just want to upgrade it to keep on playing the latest games at highest settings for one more year since my rig is 2 yr old by now. I don't think I should update my video card.



By the way, I might update my RAM. Current one is 1Gb 667 Kingston, is there a big difference with a 800? I'm currently using XP but I own a copy of Vista and I might install both so when gaming I could take advantage of the full 4Gb RAM support and DX10.



So as you can see I know little and I'd really appreciate if you could give me some advice. Again, I don't want the best, I just want to spend around 400€ (510$, I live in Spain) or so upgrading, and it's just for gaming.





Thanks a lot and take care!





John.