Palit Hd 4850 Problems

Hi, I am new here and my English is horible so,sorry about it!

My pc is :

Intel Dual Core E2180 2Ghz (2.6 Clocked)
Palit HD 4850 512mb DDR3 256bit
Asus P5B-VM SE
2X2 Gb Ram DDR2 800mhZ
320 GB Samsung
350PNF W Fortron

I changed my Hd 3650 with HD 4850 and seen some problems in almost all of the games that I play!

For Example NFS Undercover now running very bad in all ways and resolutions! And with over 40 Fps i have slowdowns nad little stops( I dont know exactly the word...)
Crysis evan with 30Fps and over i have same results like in NFS!

Do you think that my C2D E2180 is the problem nad after I upgradet 3650 with 4850 he having troubles with that?
I am about to buy C2D E8400 will this help ?

P.S I just dont get it! Why when I have over 40-50 frames I cannot play smooth! ? Thanks a lot and onse again sorry for my English. "Greetings from Bulgaria"