$300 is defiantly enough. So what you need is a little bit of RAM, a sweet GPU and then a power supply to run it all.
So for RAM, I would just add on to what you already have.
This is compatible as far as speeds and voltages go.
Then for a beastly Corsair power supply for all your needs.
This will power everything you have, and then some, with an 80+ rating.
Lastly, comes the graphics card. I see that you have an nVIDIA GPU, so lets stick with that. To maintain your budget, I would go with something like this...
GTX 460 1GB
If you want to go past your budget just a little bit, then get the same card I have.. the GTX 560 (And it does kick butt!)
Cheaper GTX 560...
Little more expensive GTX 560, great deal though.