Well to be honest id rather just build it my self…
i know it seems very scary at first cause your dealing with 1000 dollar lego for adults…
But bare with me it will be worth it.
1: you can pick anything you want for a cheaper price
2: You get the experience and can have fun with it.
3: Youll get a better understanding of whats going on inside of your comp,
if something was to fail on you.
4: Computers are often turned into a hobby, expensive hobby i might add but its just fun to be honest.
You can help your mates out if they need help or something something.
First: Amd is cheaper when it comes down to budget computers, Thats Fps per dollar.
Second: 2 cores are the same as 4, At this point. but the Q6600 is amazing at overclocking, can do easy 3.4-3.6Ghz on air.
Third: More hardware for your money by building your own.
Does overclocking your hardware shorten their lifespan?
both yes and no, depending on how much air flow you have, the Cpu heatsink will determin how much you can overclock.
It also depends on your Cpu, cause every Cpu is made different, some can only do 3.0Ghz and some can do 4.0Ghz.
As long as your Cpu isent over 75-85 degrees your good to overclock some more.
BUT i would recommend a healthy overclock with a slight volt increase and speed so that the
temp are minimum for longer periods of time. such as 24/7.
Your Pc will shut down instantly if it isent Fed enough volt as you will have to do when you increase the speed.
For 680 Dollars i found you a system thats almost the same as the deal you have found
Antec Three Hundred Illusion Black Steel
HITACHI 1TB 7200 RPM
SAPPHIRE Radeon HD 4870 512MB
Intel Core 2 Duo E7400 Wolfdale 2.8GHz LGA 775 65W Dual-Core
GIGABYTE GA-X48-DQ6 LGA 775 Intel X48 ATX Intel Motherboard (Combo deal with the Cpu)
Antec EarthWatts EA650 650W
OCZ Gold 4GB (2 x 2GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel
Why would you spend 320 dollars… on that when you have the same…
thats just what i think, Nvidia gpus arent for budgets when Ati`s cards are more fps per buck.
The Case: Good air Flow, With 2 120mm in front Pushing cold air inside the case… 1 120 in the back and 1 120mm on the top blowing out warm air. This is great Positive air pressure.
The HDD: 1Tb? enough? well yes it is! 
hehe 7200 Rpm is a must. a nice drive.
Video card: Fine, it might not be GTX 285 material but it gets the job done and isent to far behind it, plays any game out there over 60 fps on highest settings on 20-22 inch screen. Except a handful like crysis and crysis… 
but there not really games as much as there more a benchmarking tool for seeing how far your comp can preform…
but there good games no doubt.
Cpu: 2.8 Ghz normal, healthy overclock at 3.5-3,6 i would guess… if not more.
Motherboard: Supports up to 1200mhz DDR2, Maximum Memory Supported 8GB, 2 PCI Express 2.0 x16 slots, so you can add another 4870 or even a 5870 when that time comes and you want your lovely DX11 grafics and gameplay :)… and yes you can do 5870 + a 4870 together, so no worrys
Power supply: Manufacturer Limited Warranty: 3 years. SLI Certified CrossFire Ready 80 PLUS Certified Active,
Witch means your power supply will last long, very efficient, can handle 2 Video cards.
Ram: 4Gb, 800Mhz, good quality ram, have it my self so i can say its good… can overclock it to 950+mhz if you want.
WOW WOW to much info man! i better stop it dont wanna scare you away now hehe
hehe 7200 Rpm is a must. a nice drive.
but there not really games as much as there more a benchmarking tool for seeing how far your comp can preform…