If you have 900 to play with you can still get a prety good single card, try a 4870/5770you should be able to fit that in your budget. also, you only dont save money when you build a computer for just casual use, ie. web browsing, word documenting etc. For PCs above 400ish you will get so much more than a shop bought one. Building a computer is easy really, kinda like lego, everything just pushes together.