External GPUs?

I have had a quick look online but you lot are more techy than me.

I have been waiting for a while now for the new iMac's to come out and during this waiting it has made me wonder whether I should get a Mac at all, I play a lot of games (mainly on 360) and would like to play more on the PC. I know that the base iMacs atm can handle Skyrim and Battlefield but I'm thinking now that if I get a PC I can do all the stuff I can do on an iMac on a PC plus have the upgradability with graphics cards. The only problem there is I have never built a PC before and I kinow if I want a "gaming computer" it is best to build it myself to get more value for money, but I do want an iMac, so my main question is....

Are there such things as external graphics cards, for example through Thunderbolt or not? Would be interesting to know, and it would swing me back to the apple side of my decision.

 

Just built a PC on ebuyer,

6 Core 3.3 Ghz Processor
8GB Ram
AMD Radeon 77701GB
1TB HDD
LiteOn Disc Drive

All this plus delivery for £663.07

As my name suggest I really don't know what to do anymore.

If I bought an Imac do you think it will retain its value enough over the coming years to sell it and then put like £400 towards getting the next one which will then put me on a level playing field to play that generation of games?

 

Okay may go for this instead, will probably buy it in the next week:

Processor - Ivy Bridge
Intel Core i5 3570K 3.4GHz Socket 1155 6MB Cache Retail Boxed Processor (170.06)
MOBO:
Asus P8Z68-V LX Socket 1155 Onboard graphics output 8 Channel Audio ATX Motherboard (74.72)
CASE:
Coolermaster HAF X Case (129.99)
PSU:
OCZ ZX Series 850W 80+ GOLD PSU Fully Modular PSU (109.98)
RAM:
G-Skill 8GB (2x4GB) DDR3 1600Mhz RipjawsX Memory Kit CL9 (9-9-9-24) 1.5V (39.93)
GPU:
Gigabyte GTX 560 OC Edition 1GB GDDR5 Dual DVI Mini HDMI PCI-E Graphics Card (137.97)
HDD:
Seagate 500GB 3.5" SATA-III 6Gb/s Barracuda Hard Drive 7200RPM 16MB Cache (54.54)

OPTICAL DRIVE:
DVD Drive (14.87)

TOTAL:
731.86