Low FPS in Source based games

Hello,

Well having spent quite a bit of green for my age getting a new ATI HD4850 I seem to be stuck with this problem.

My specs are:
AMD x2 3800+ S939 @ 2.3 (Giga-byte mobo no have voltage but my A8N-E still has the same problem)
Gigabyte GA-K8N51GMF-9 (Latest BIOS)
4 Gigs kingston value ram (@1:1 in overclocking gets to 220 before ram gets unstable)
120Gig Maxtor 7200RPM
450 Watt RHINO Panther PSU (Cheap but its quiet and cool so I dont think im stressing it)
HD4850 (ATI virsion I think)

I am aware that my CPU is a bottleneck but I hear its not by much. According to some other sites at 2.5-2.7 I should be able to reduce it to a minimum or even remove it in most games.
When ever I look down long A in Dust2 or even inferno in L halls in CS:S my framerate drops from about 150FPS(maxxed out settings ) to about 50-60FPS. Im not shure what to make of this problem because from my knowledge that should not happen, so im pointing the finger at my CPU but before I spend even MORE money I though i'd ask you guys.
What do you think? Ive even gone to about 2.6 on my stock cooler with about 1.47Vcore at a max of 58C just to see if it would help but it did not really help(around 5 FPS increace).

Thank you guys for helping as I want to try and make some $$$ at ESEA