My current system:
intel core 2 duo E8400 3ghz
Sapphire Radeon HD4870 1gb
G.Skill 2x2gb ddr2-1066
Gigabyte EP43-DS3L lga775
1. Will I see a big difference in upgrading to a quadcore? My system is used mostly for gaming, and it seems that there still aren't many games that actually use more than 2 cores. I'm worried that I might actually see a decrease in performance if I get a quadcore with a lower clock speed.
2. Which chipsets are going to be upgradeable several years from now? I really like the performance/price of the AMD Phenom II, but would it be worth the extra cost to get an i5 and upgrade to an i7 800 series in the future?
3. Should I wait for Bulldozer or Sandy bridge? I only have ~500$ to spend on upgrades, so I'm not sure they will be affordable when they are released.
4. Or should I just spend ~60$ on cooling and learn how to overclock
The gpu still seems to be keeping up well, so I don't plan to upgrade it for another year.
You will see an increase in performance upgrading that 775 platform to a quad core, but you are probably better off spending that money on a new build early next year following the release of Sandy Bridge. Yes on #3... Come back to this question in Feb / Mar and things will be different regarding price / availability. #4 is up to you, but putting that $60 toward a quad core is probably a better spend over a new cooler (if you want to spend money). IMHO... stick with what you have for now.
LGA1156 is "dead" so there is little upgrading down the road past what you can buy today. LGA1155 (SB) may offer that option if it lasts more than two years before Intel switches platforms again. Not sure...
If you are looking to build a system with a PLANNED CPU upgrade in 2 or 3 years, the AM3 platform is a good option. AMD will still be making the Phenom / Athlon AM3 processors through 2011, and maybe further, which makes that 6+ core upgrade in three years easy with the MB you buy today.