Ok, I have embarrassingly archaic hardware. I used to have a beefy gaming PC but had to sell it a few months ago, now I'm using a frankenbeast of a PC that I built from old parts I had as well as some pieces that were given to me. I really want to play minecraft but with my current setup it's really laggy.
What I have is all circa 2005 stuff, a socket 939 mobo currently with a sempron 3200 1.79 GHz (LOL i know), integrated ATI radeon 200, and 2 GB of DDR memory. I have an available PCIe slot, 1.0 but it still allows for expansion.
I can get an Athlon 64 x2 CPU for about $35, or spend a bit more and get the 4400. I can get an older but decent GPU for probably about $50. I can only afford 1 of these upgrades now and will have to wait until I get my next pathetic check to get the other. Which should I buy first.
And don't tell me to build a new PC, get a new mobo, etc. If could, I obviously would...but that's not in the card any time soon.
I'd say spend around $60 and get something like an HD5670. Upgrading the CPU is pointless anyway because by today's standards you are going from a old dinosaur to a new dinosaur. you can save up and go for a new build. The GPU you can carry into the new build.
What are you doing with this computer, gaming or intensive work like graphics rendering? If it's gaming then I would say go with the graphics card first. If its something that is CPU intensive then go with the CPU first.
Well since your already spending about $90 you could by a 40 dollar am3 motherboard with a 50 dollar athlon ii x3 on ebay. The motherboards integrated gpu will also be an upgrade. It isn't worth spending money on really old components as zero said since you'll still be behind.
DDR3 is extremely cheap. For 25 dollars on ebay you could 4 gb. And for 50 dollars you could get a 4850/4870/250 gts, which can all play battlefield 3. This all comes to about 155 dollars, 115 dollars if you stick the intigrated. You'll also have decent upgrade options for the future which is nice. Be patient, biding will be your best freind since your trying to get the best out every dollar.