Well I would upgrade your PSU while you are at it as well, at 500w you are not gonna get any much higher end GPU.
I would suggest http://www.newegg.com/Product/Product.aspx?Item=N82E168... . It keeps the 2.0x16 setup and offers a whloe lot more bang for your buck, along with being under your budget to upgrade your PSU. I would also upgrade your RAM to http://www.newegg.com/Product/Product.aspx?Item=N82E168... It wouldnt put you over 300 pucks for the whole upgrade.
GPU- Radeon HD 6970
PSU- SeaSonic M12II 620 Bronze 620W
RAM- G.SKILL Ripjaws X Series 8GB (2 x 4GB)
I hope this helps
What you should look at is a GTX 760, it's probably going to look like you just went 5 years in the future by changing to a better GPU. Overclocking the GPU is something that takes 2 minutes, literally, and is not bad at all. I totally recommend it, just don't go crazy, you can download EVGA Precision X, or MSI Afterburner, then just add +80Mhz to the core clock, and +200Mhz to the memory clock. Simple, you just slide 2 bars. Then, increase fan speed to 40-44%.
the 6970 is about 160, its about 4X better then your current one and with free shipping but im not sure if thats for the US or everywhere. As for the PSU, you will want at least 80 plus certified, the one i recommended is 80 plus bronze and is semi modular. That means its easier to connect all your power connections and looks cleaner but you dont need it. YOu can get a 80plus Gold non modular and save some bucks. Overall I would recommend at least 600W-700W you dont have a super strong computer but if you want a better GPU then you will have to get a better PSU.
I hope this helps and hope the free shipping from newegg applies to everywhere.
I am afraid almost no place has free shipping this far out I checked with Newegg.
The prices I specified is a shop really close to me and it is convenient.
Could you tell me how the low range cards will do from the list below if they're half decent I can get the R1000 PSU 600W it is 80+ BRONZE (And my brother tells me it has a 5 year warranty or something).
that sucks, well in that case i would go with the MSI Twin Frozr HD 7850 2GB 256bit GDDR5 OC Edition Graphics Card or the SAPPHIRE Radeon HD 7870 GHz Edition 2GB 256bit GDDR5 OC Edition Card. Might cost you a little bit in the front end but it will last you for a while.