If you buy only the components you need, you won't have to sell as many things and you will save a lot of money. Pre-assembled computers are usually not a very good deal since they always charge you a lot extra for them. The computer you chose is quite good (except for the CPU which I'm afraid might be a bottleneck while running certain games like FSX... I would get a slightly better one) but I think you can build one just like that by a much better price.
From your config I think say you won't need new drives nor sound card nor mouse/keyboard. You can go check some stores and get a good CPU, PSU, some ram (depends on whether the 2Gb you have are slower or not DDRII... if they are alread DDRII at like 600MHz, then you won't even need more RAM... unless you wanna go 4Gb) and a new grafics card like the 8800GTX. You will also need a new motherboard... probably whith an Intel chipset (if you don't think about going SLI anytime soon) or a nvidia n680i (if you wanna go SLI).
I say it again... if you don't wanna assemble a computer yourself, then the one from the link is pretty good. If you could get one with a batter cpu that would ge great.
