Is this a decent gaming system? any Improvemnets?

[NL__Fool]

I want a new computer and I have done a bit of research.
How does this rank as a gaming system?

Any impovements or suggestions that you have?

Intel Core 2 Duo E6850 3.0GHZ
Inetl D975XBX2
Coraise 3 GB DDR2-800
Western Digital Raptor X 150GB Serial ATA150 (10,000RPM)
Nvida Geforce 8800GTX 768MB GDDR3 Video Card

Some questions

How much of a difference will I see if I upgrade my RAM to the DDR2-1066? It will cost around $100 more

How much of a speed difference will I notice between a 7,200 RPM or a 10,000 RPM Hard Drive?