Tough decision.

Hello everyone. I built my system 6 months ago and i want to make an upgrade for either my GPU or CPU with a budget of $300 total that i can buy at Fry's electronics (only computer store close to me right now and my paypal is down for online payments and i dont want to use credit cards)

Current system:

Mobo: MSI p67a-c43 (b3)
CPU: i3 2120 2nd generation (sb) running at stock 3.3ghz. Stock fan.
GPU: MSI GTX 550ti Cyclone OC (oc is factory clocked)
RAM: CORSAIR Vengeance 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Low Profile Desktop Memory
HDD: Western Digital Caviar Green WDBAAY5000ENC-NRSN 500GB SATA 3.0Gb/s 3.5" Internal Hard Drive
Case: Full ATX Case (unknown brand or model, its just this metal white case i got from a local computer store for free)
PSU: Antec NEO ECO 620C 620W Continuous Power ATX12V v2.3 / EPS12V 80 PLUS Certified Active PFC Power Supply
Monitor: Acer S200HLAbd Black 20" 5ms LED Backlight Widescreen LCD Monitor 250 cd/m2 ACM 100,000,000:1 (1000:1)

So basically im looking to upgrade my system with the money i have. I think i want to upgrade my CPU to a i5 2500k and perhaps oc it to 4.5ghz. Although maybe an upgrade to my video card is better now and i could get the CPU later? DO i need an upgrade to my PSU first so i dont need to worry about it later? or a complete new mobo? A new monitor isn't really something i want because i like the current resolution it gives me.

Im a hardcore gamer and always trying to get the best visuals while also using it for 3D programs liek Maya. But i don't get much money with the odd jobs i get. SO long term use should be kept in mind.

What do you guys think i should do with this money for my system?

PS: need more information just feel free to ask.

EDIT: Put this thread in the wrong place, hope its ok :{