your computer is pretty good... no need to upgrade now...
just wait like 1 year, then buy a nice comp for like $2000
with $2k next year you'll probably get quadcore cpu and a dx10 gpu..
upgrading to a c2d or w/e now isn't really necessary... best bet is to wait 1 year, by then your computer will be mid range.
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius