Why does minecraft lag for me? - 1

My specs:
AMD fx 8350 4ghz 8 cores
8 GB of ram 1666 mhz DDR3
Nvidia GTX 450 512 mb ram
Windows 8 64 bits

My question is why does minecraft lag for me? I usualy get around 5-10 fps on modded minecraft and maybe 20-30 fps on vanilia minecraft. I have allocated 2 gigs of ram to the java machine but its still not working like its should.

GTX 450? That's weak…

Ik but i have friends who play minecraft with integrated video and they still get better fps than me