[b0unce. [?]]

I'm looking upgrade my incredibly old desktop. This used to be a machine I used for gaming, and didn't have any issues running most games at the time, after maybe 4 or 5 years I'm looking to upgrade. I recently had a bit more time to play some video games so I picked up CoD:Black Ops and I'm going to grab Dead Space 2 pretty soon. I noticed for the most part CoD runs somewhat smooth, I get the occasional frame rate drop and it gets a bit choppy (these are with settings set at about medium level. it made no difference when i lowered everything).

The only parts that are somewhat recent is my video card (about a year old) I think. Anyways currently this is my setup.

amd phenom x4 940 3ghz
asus m3a78-em amd am2+ mobo
4gb ram. x2 2g ddr2 sticks
xfx radeon hd 5850
win7 os
ocz modstream pro atx 700w psu
atx case
stock heatsink, top and side panel fans.


Thats probably the key components thats needing to be replaced.
question is, is it worth replacing a the processor, mobo, and ram and just run with the vid card I currently have? Or start from scratch and build a new desktop. I don't necessarily have a budget, but I wanna say I would roughly want to spend around $500 - $600 give or take if I were to replace/upgrade parts.

Anyways I'd love to hear your input on this.
Cheers