Heres the specs:

OS: Genuine Windows Vista Home Premium (64-bit)
Processor: Intel® Core™ 2 Duo processor E8400 (3.0GHz)
Memory: 3GB DDR2-667MHz dual channel SDRAM (2x1024,2x512)
Graphics Card: 512MB NVIDIA GeForce 8600GT, TV-out, DVI-I, HDMI
Hard Drive : 500GB 7200 rpm SATA 3Gb/s

Includes a 20 in' monitor.

Price: $1,200

I was considering OC'ing the Processor and Video Card if I need to.

Way overpriced in my opinion, and why the 8600GT?

Buy an 8800GT, that processor is a nice overclocker, be sure to replace the stock HSF.
I worked out an 8800GT build to be around $954 - OS?

Can you give us anymore info on the build? What site is supplying you that information you may not even be able to OC that (if its a dell)

Woops, forgot to add that also includes a 20 in' monitor.

That explains the missing $200, is this a prebuilt we're talking about? where you getting it from

There's no overclocking on the VAST majority of pre-built systems. Some "enthusiast-grade" computers (pfft--like Alienware, VooDooPC) charge a HEFTY price premium to overclock it for you--not a good idea really. If you want overclocking yourself, build it yourself. A few points I wonder about... why only 3GB of RAM on a 64-bit OS? That only makes sense on a 32-bit OS, as 32-bit applications can use a maximum of 2GB and that extra 1GB for the OS & background processes. If you use even 2 sticks of RAM--go with a 2x2GB (4GB) set of RAM, so you can have a future upgrade path with that 64-bit OS, as there will inevitably be 64-bit programs that run much better with 8GB of RAM, versus 4GB of RAM.

I'd consider the 8600GT to be too little to game on a 20" monitor--if you plan on gaming. The GeForce 8600GT is aimed at budget gamers with a smaller-than-19" monitor. The Radeon HD 2600 Pro/XT and HD 3650 are aimed at HTPCs and budget gamers. If you want to play on a 20" monitor, look for a GeForce 8800 or Radeon HD 3800 at a minimum. Go for 512MB of graphics RAM--when I got my HD3850, the 512MB versions were JUST coming out and at a $30+ price premium and with supply shortages, compared to the $10-$20 premium they're at now and with large supplies. That, and the card I have dropped $10-$20 in the past 4 months.