well first off, in your case I wouldn't even consider tne brand named computer, your going to pay more for less performance than you would in a custom. I probably don't live in your country but I can recommend some parts that will work well for you.
for the graphics, go something like 6600gt, 6800GS, GT or Ultra, 7800GT from worse to best, I wouldn't recommend ATI, NVIDIA is easily dominating the market at this point and thier cards perform superior.
For processor, you should go for an AMD64 at lowest and you may even consider dual core. 3700 San Diego is a good place to start.
Make sure you get a quality motherboard like DFI, I like gigabyte and ASUS whcih have been reliable for me so you might want to consider them as well.
you should aim to get 1gb of DDR400 memory, some quality modules can be found from OCZ, G skill, Kingston for lower budget.
Just make sure your board has plenty of features, a custom built made properly has just about as much chance of error as a brand, the hardware doesn't what issues you get in software, merely increases performance.
And secondly I don't know anything about brand computers, I had a HP which was alright but for 3000 dollars didn't really cut it.