Build a PC!
You can def do it if you reused some of the parts from your current PC.
Re-use the: CD/DVD $30-39.99, Hard-Drive (as storage) $79.99, Monitor ~$100.00, Mouse + Keyboard $45-60, could probably re-use the OS but since you'll be pushing 8GB ram you'll want to run 64Bit OS.
There you go you've saved minimum $250
If you have a microcenter in your area I would shop there for your motherboard + CPU combo as they have really good deals on them (usually $40-50 bucks on a bundle deal, sometimes more, up to $69.99)
Do you want to SLI in the future? If not you can go with a Micro ATX board which will be significantly less than a standard ATX board and save you about $30-$45 bucks depending on what bells and whistles you want in a MOBO (don't care to use the USB 3.0 in front of the case? Then get a board that doesn't support it, bam instant savings lol). <---- seriously though!
EVERYTHING ITALICIZED IS DEPENDENT ON THE TYPE OF BUILD YOU WANT / FUTURE PLANS.
CPU - AMD Phenom II X6 (series 1000 area) / 1045 cost $99.99 @ Microcenter (it'll perform well enough for your needs and save you $ rather than going with a Intel chip. Nothing against Intel I'm just recommending savings). If you want you can go down to a Phenom II X4 BE (Black Edition) and save a little more on only 4 cores over 6. You can always up the speed by overclocking it.
I would stay away from the FX series of AMD chips - performance is not there
MOTHERBOARD ASRock 970 EXTREME3 - $89.99 (both do not have front USB 3.0 support)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
MOTHERBOARD *If wanting to SLI with @2x16* ASRock 990 EXTREME3 - 129.99 (both do not have front USB 3.0 support)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
RAM - get @ least 8gb with $60 budget & make sure you choose something on the manufacturer qualified list to save yourself from future headaches. 1333 speed for 1st board / up to 1866 for 2nd board
SSD (BOOT DRIVE) - 32GB Kingstone - $59.99 / 44.99 after rebate (use this to install your OS and watch it boot up and shut down fast as lightning!)
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
STORAGE DRIVE = re-use your previous HD. Everything other than installing the OS goes here
CASE = Budget it between $80.00-$100.00 Antec NineHundred or Carbide 400R are good choices (there are a lot of other options just look for something with lots of holes in the tray for wire management)
http://www.newegg.com/Product/Product.aspx?Item=N82E168... = Antec 900 (OVER 5k 5 EGG REVIEWS)
http://www.newegg.com/Product/Product.aspx?Item=N82E168... - Corsair Carbide 400
POWAAA SUPPLY - $69.99 - $120.00 This can be tricky (not really) but depending if you want to SLI in the future or not you may not need a top end power supply. Also do you want full modular support (do you want to be able to only connect the necessary plugs needed? This is especially nice for wire management and case cleanliness). Full modular supplies are more costly than none full modular.
OS -Windows Home Prem 64 Bit $99.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Now you have aprox $391.00 where you can ask the people on these boards for help and recommendations on a video card. You can get a good GPU for $260-$$310 that'll suit your needs easily. You can also get an aftermarket CPU cooler to keep your CPU cooler which will open more opportunities for overclocking.
Also the reason why I suggested and AMD Phenom chip over Intel is once again price. You can spring for a 2500k but it'll cut $80 into your savings and may increase the price on your motherboard (not likely but could). You said you wanted to play Diablo 3 and that is GPU intensive. I'm not sure what type of hog Guild Wars is.