Depends on how good you want it to look. I'm going to be upgrading at the beginning of August to the following:
Intel Q6600/E6750
4GB DDR800
Geforce 8800 GTS 640mb
Windows Vista
This setup should allow DX10 mode on any of these games at fairly high settings. Your current box is definitely lacking for any of these games. I can post a Newegg wishlist for the individual parts if it's of any interest - at the beginning of August, this kind of box should be buildable for ~$1500.
I just upgraded last month to the following:
Intel Core2Duo 6420 (slightly lower clock, for MB of cache, very nice and affordable)
2gigs PC6400 DDR2
GeForce 8800 GTS 320mb
Windows XP Pro.
Plus a gigabyte brand motherboard of some model I cant remember right now...
The whole upgrade cost me like $850 (I bought a DVD burner and power supply also, forgot about those) and has totally changed the way I look at games, Frame-rate wise and graphically... its crazy.
add on Vista and i will be DX10 compliant but for now, Im not ready to pull the trigger on Vista, too many reported issues. Maybe when the first service pack comes out or some new DX10 games is released that I cant live without... you never know.