Update your video drivers
Defragment your hard drive
run CCleaner
Disable AA
Also, what settings are you running it at? Enthusiast? gamer? They make a big difference.
8600GT eh? hmm. Tell me the name/model number of your powersupply, it may be possible for you to add a new GPU in. Also supply the specs for your motherboard.
I'm guessing it is a prebuilt?
1 GB...I'd get another GB. RAM is cheap and that may make a big difference in your case.
EDIT: I really nice upgrade option for you would be to purchase a 9600GT. They sell for just under $100 online and check out these benchmarks...
http://www.tomshardware.com/charts/gaming-graphics-charts-q3-2008/compare,747.html?prod[2067]=on&prod[2086]=on
Your current card is on the left. The 9600GT offers about
3x the performance in most games. Another benefit of the 9600GT is that it is the best value when 2 are combined in SLI. Typically 2 cards in SLI or Crossfire might offer a 20-40% performance boost over a single card. 2 9600GT's in SLI, however, break the mold and can achieve 80-90% extra performance over a single card. That is incredible value.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=2010380048%20106792522%201067937365&bop=And&Order=PRICE