The point is..... these computers are already built, and you are not allowed to swap and change components.
In that case, I personally would go for something else... the CPU and GFX card maybe better than your current ones but they're still relatively low end by today's standards, so it'd be a double expense to have to upgrade them later.
Instead of buying a prebuilt, maybe you could look into doing a custom build yourself, or check out your local PC stores if you're not up for it yourself... either way, you could save yourself a bit of cash, and you get none of that pre-installed crapware/bloatware which comes standard with proprietry brands
On the subject of AMD quad cores, they started coming into their own more with the Phenom II series, so if you're going with an AMD , the Phenom II x4 920 or 940 is a decent CPU that'd do the job for you.
As for a GFX card, the Galaxy 9800GT 1gb (Low Powered) is a decent card and quite a bit cheaper than GTX200 series cards. BTW, low powered doesn't mean that it is somehow under performs, but that it draws power from the mobo and does not require a power connection to the PSU... meaning it doesn't tax your power supply and is somewhat shorter than most cards these days, thus fitting into a standard case more easily.
Anyhow, all the best with getting what you want/need.