To be honest with you trencin, your current computer seems to be good for what you want to be doing right now, mostly the office software and some light gaming. But if your serious about upgrading I do have a few suggestions for you.
my best guess would be to go 939 with this setup so you dont have to get ram
Price ~$300 shipped from newegg.
Well first things first, getting a 939 or am2 board with an agp slot is a bad idea, that means your tied down if you want to upgrade again, and since it seems that your a little strapped for cash and will be upgrading incrementally, I think its very important that you keep your options open with a nice motherboard. Newegg is having some great combo sales on 939 boards and processors
AMD Athlon 64 3200+ Venice 2000MHz HT 512KB L2 Cache Socket 939 Processor - Retail
+
ASUS A8R-MVP Socket 939 ATI Radeon XPRESS 200 CrossFire ATX AMD CrossFire Motherboard - Retail = $142
As for a decent case with a power supply
Linkworld 435-10 C2228 Black & Silver Steel/Plastic ATX Mid Tower Computer Case 430W Power Supply - Retail = $25
And because you need a pci-express card
POWERCOLOR X800GTO256MBDDR3 Radeon X800GTO 256MB 256-bit GDDR3 PCI Express x16 Video Card - Retail = $110
So that all comes out about $275, it estimates my shipping as $25 so yours would be close to that, $300 and youve got a good START on a good computer. I know thats a little over your budget but I think every bit of it is necissary, you might even want to consider new RAM to go with it.
So if you decide to upgrade I would go on a path similar to that one, but in all honesty, I would say your fine with what you have right now, save up a little more and do a complete new build. Good luck