I'm getting sub-par performance!

[NeptuneUK]

Running an AMD II 640 Quad clocked at 3.15ghz
1x 4Gb DDR3 @1400mhz
GTX 570 1280mb stock
X-fi gamer
Asus M4A77T
750W Thermaltake PSU

My 3dmark 11 score is P4200 which it is telling me is low for mystem. I've seen people with similar systems getting around 5400-5600 scores!!
I need to find the culprit and sort it out, where would you recommend starting?

I've already checked temperatures, no issue.
Made sure all drivers are installed and up to date.
Nothing running in background.
The only things I can think of would be the single RAM stick or PSU not cutting it. I know the PC specs are hardly cutting edge but the PC is really struggling with stuff like Starcraft II.

 

[NeptuneUK]

Thanks for the replies guys. I'm looking at an upgrade in the next month and was unsure what the biggest bottleneck is.
I'll look at going SLI with the 570. Get some performance RAM and check the phenom II prices (which seem very reasonable!)

edit: could anyone recommend a reasonably priced AM3 SLI motherboard? (Mid-range?)

 

[NeptuneUK]

Spot on. Thanks mate. I was browsing through the mobos with little idea to the required chipset.