$65 for two gigs? I paid about $150 for the two gigs of G.Skill I have when I built this thing. DDR is higher than DDR2, but it still is reasonable.
Grab two gigs in addition to what you already have.
If you can find a decent deal on a dual core CPU, grab it. Check eBay, for starters.
You are running a 6800gt. Decent GFX cards can be had fairly cheap, around $100 for a 3870, 9600GT, 4830.
Hard drives have got crazy cheap nowadays.
Again, it depends on your budget. Since you have the basics already (case, PSU, hard drive, opticals), a small budget may let you go with a newer generation of internals.