Need Advice on Upgrade

Hey Guys,

When I bought my computer during the spring of last year I asked for the advice of the users of Tom's Hardware, I was not disappointed with the result. Anyways, my birthday is coming up and I've decided to buy myself a $300 upgrade for gaming purposes. I was wondering if you could give me some opinions as to what I ought to upgrade (I've listed the guts of my machine below) and perhaps some suggestions as to what I ought to replace the aforementioned part with. I'm extremely grateful for all your expert opinions and if you want to know any specifics about my machine let me know.

[Edit] I suppose I'd like to find out what you guys feel the weakest part of my system is (in terms of gaming). I don't have a sense of wear the components stand in relation to price and worth of the upgrade.

MS Windows XP Home SP2
INTEL Core 2 Duo E8400 3.0GHz FSB1333MHz 6MB LGA775 Dual-Core
ASUS P43 P5QL-PRO 3PCI/ 1PCIE X16,4D.DDR2-1066MHZ 1333 /1066FSB,SATA GbL
OCZ XTC FATAL1TY OCZ2F8004GK 4GB DDR2 2X2GB DDR2-6400 800MHz
EVGA 9800GT 512MB DDR3 SUPERCLOCKED 2DVI PCI-E Dual DVI TV-out
WD Caviar SE-16 / SATA-II 5000AAKS 500GB 7200RPM, 16MB, SATA-II
LG GH22NS30 SATA BLACK DVD±RW (22X+/-) DUAL LAYER w/ SW
Coolermaster eXTREME POWER+ 500W (RP-500-PCAR) 24/20PIN

Hey there,

1024 x 768 is pretty much what I keep it at, the monitor is a BenQ FP737. It's about 17/18inches. Well, I was really disappointed when I bought Assassin's Creed for the PC (It was $5 on Steam a couple of days ago, I thought what the hell). On full settings it'd wank out and I'd get like 20-30 FPS. I played WoW for awhile on this machine and I couldn't max that out either. I'm going to be playing Borderlands but generally I am just looking to make my PC a little more powerful to keep up with the games.

9800GT and you couldn't get it to play well at 1024x768? wow, I though 9800GT was pretty decent. I didn't think WoW was that GPU intensive.

The 5800 is probably overkill at that resolution. To tell you the truth I'd be tempted to go for a new monitor! Also, your PSU (as bliq pointed out -good shout!) is probably going to come up short on a graphics card much more powerful (only 1 PCIe and 16A on each 12V rail as far as I can find out).

Other people on here are probably going to know better so I'll wait on them for power recommendations.