Is it possible to build or buy a PC...

Building a PC from scratch is usually good for overclocking. If your not interested in overclocking then this pc is reccomended because I have personal experience with it and its VERY positive, just upgrade the video card

http://www.circuitcity.com/ssm/Specifications-of-HP-Pavilion-Desktop-PC-A6109N/sem/rpsm/oid/185481/rpem/ccd/productDetailSpecification.do#tabs

Building would be your best choice. That HP isn't bad for the price, but it's a bit weak in the processor department, and would need a GPU.

amd 4000+ x2 $70 http://www.newegg.com/Product/Product.aspx?Item=N82E16819103774

gf 7900gs $140 http://www.newegg.com/Product/Product.aspx?Item=N82E16814186013

or 7950gt $190 http://www.newegg.com/Product/Product.aspx?Item=N82E16814143090

2gb ram $35 x2 http://www.newegg.com/Product/Product.aspx?Item=N82E16820146580

ASUS M2N-MX SE Socket AM2 m/b $55 http://www.newegg.com/Product/Product.aspx?Item=N82E16813131183

320gb sata hdd $80 http://www.newegg.com/Product/Product.aspx?Item=N82E16822148140

dvd writter $33 http://www.newegg.com/Product/Product.aspx?Item=N82E16827248006

case $16 http://www.newegg.com/Product/Product.aspx?Item=N82E16811147073

500w power supply $60http://www.newegg.com/Product/Product.aspx?Item=N82E16817153052

so $524-$574 for a system that should run it pretty well add windows for another $100

I also recommended in building a PC, in order to collect all the best thing in one PC.

In addition if you want to upgrade it later you can change any part on it easily.

(also if there a part get damage and you need to replaced (using the warranty) you have only to take that part, but in the un-build PC you have to take the whole PC).