There are various guides on this site and elsewhere on the internet for overclocking the core 2 duo cpu's. Also you can google "e6500 overclock" and probably find other people that have done it and will give you their exact settings. Your chip might or might not overclock as well as someone else so keep that in mind too.
That cpu has a multiplier of 11 so it should be relatively easy to overclock. You may or may not need a better cooler, depending on how much overclock you want. With only 2GB of ram and Vista, you might get a performance increase just by adding some more ram.
To overclock your cpu- let's say you would like to hit 3.3 GHz which should be very easy, you would set your system bus to 300 MHz. You will also likely have to reduce the ram speed in BIOS to as low as it can go, at least to start with. You may or may not have to set your PCI bus to 100MHz. Then crank it up and see what it does, run free programs like coretemp to monitor the cpu temperature and prime95 to put a good load on the system. If the temps don't go too high, then you can raise the speed a little more. I would guess that you have a good chance of going to 3.5 or 3.6GHz, which would mean system bus speeds of 318 to 327MHz.