With your current Mobo you should go for a CPU+Mobo upgrade as I checked the spec of your Mobo and they don't look good for a Gaming machine. In 15k you can get following:
Intel� Core2 Duo E8400 (3GHz) Processor Rs9,735/- (Local prices may vary)
Abit IP35-E Motherboard Rs5.835 (Local Prices may vary)
Total=Rs15,570
If you can stretch your budget 1 or grand more you can get:
Intel® Core2 Quad Processor Q6600 Rs11,055
MSI P35 Neo Combo-F Motherboard Rs6,010
Total=17,065
I personally would opt for MSI one bcoz though the Abit Mobo supports upto 8GB RAM whilst MSI one supports only upto 4GB but it has support for both DDR2 667MHz and 800Mhz and even upcoming DDR3 RAM bot 800MHz and 1066MHz which I say makes it very future proof.