Any upgrades possible?

[endershero]

So I built my comp which is mainly for gaming from this site about a year ago. http://www.squidoo.com/electronicandmore. The rig I have now is as follows:

G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory Model F3-12800CL9D-4GBRL
ASRock 870 EXTREME3 AM3 AMD 870 SATA 6Gb/s USB 3.0 ATX AMD Motherboard
HIS H687F1G2M Radeon HD 6870 1GB 256-bit GDDR5 PCI Express 2.1 x16 HDCP Ready CrossFireX Support Video Card with Eyefinity
Antec NEO ECO 520C 520W Continuous Power ATX12V v2.3 / EPS12V 80 PLUS Certified Active PFC Power Supply
AMD Phenom II X4 955 Black Edition Deneb 3.2GHz Socket AM3 125W Quad-Core Processor HDZ955FBGMBOX

It runs all the games I play fairly smoothly. No major issues and I'm very happy with it but I was wondering if I'm able to upgrade any of the parts for it. I figure its a year old so I thought if I could get more out of it, why not.
I also am thinking about buying another GPU with Crossfire support but I would like to get some opinions on that first. Any help will be appreciated.

 

[amk-aka-Phantom]

You will need a bigger PSU for another card.

You could upgrade your CPU, but its no use, yours if just fine.

You could add more RAM, but then again, if games run smooth, why bother?

 

[endershero]

Thanks for the reply. I was thinking of upgrading the RAM to an 8GB and i was wondering if there's anything special I should look for? As for buying another GPU for Crossfire, I guess I can hold off on that for now.