Gateway DX 4200 09 - What would you upgrade with $300??

Hey!

I have $300 bucks to use on my computer upgrade. I seriously want a better video card but I don't want to bottleneck the performance becayse of the CPU. Could I upgrade GPU and CPU processor for this amount? What would you get? GPU, Power supply, CPU? What combination and maybe if you could recommend which one specifically 

I will do some gaming on my 100'' 1080p projector screen. I mainly play World of Warcraft now, but I want to be ready for Diablo 3 and maybe Crysis.

My current system:
Gateway DX 4200 09.

Processor
Type AMD Phenom X4 9100e / 1.8 GHz
Multi-Core processor technology Quad-Core
64-bit processor Yes
Installed Qty 1
Max processors supported 1
WIndows Vista 64

Cache Memory
Type L2 cache Installed Size 2 MB
Mainboard
Chipset type AMD 780G
Data bus speed 3600 MHz

RAM
Installed Size 4 GB / 8 GB (max)
Technology DDR2 SDRAM
Memory speed 800 MHz
Memory specification compliance PC2-6400


Expansion / Connectivity
Expansion Slots Total (Free) 1 ( 0 ) x Processor - Socket AM2 , 4 ( 2 ) x Memory - DIMM 240-pin , 1 ( 0 ) x PCI Express x16 , ( 0 ) x PCI Express x1 , 1 ( 0 ) x PCI Interfaces 1 x Mouse - Generic - 6 pin mini-DIN (PS/2 style) , 1 x Keyboard - Generic - 6 pin mini-DIN (PS/2 style) , 6 x Hi-Speed USB - 4 pin USB Type A ( 2 in front ) , 1 x Display / video - VGA - 15 pin HD D-Sub (HD-15) , 1 x Network - Ethernet 10Base-T/100Base-TX/1000Base-T - RJ-45 , 1 x Modem - Phone line - RJ-11 , 8 x Audio - Mini-phone stereo 3.5 mm , 1 x Audio / video - HDMI - 19 pin HDMI Type A , 2 x IEEE 1394 (FireWire)

Power
Power device type Power supply
Power provided 300 Watt