Well, AMD is planning it's lauch of next-gen hardware at the end of April. They plan to announce their GPUs as well as future plans for processors/sockets. So by mid to late May we will probably see price competition driving prices down, as well as a firm statement on upgradeability. Personally, I am waiting until then. It is also probable that more drivers will be created for Vista as well, making future compatibility more feasible.
As for the computer....
Does it come with an optical drive?
What OS does it come with?
The processor is actually an AMD Athlon 64x2 5000+. No, it is really not on par with Core 2 Duo as far as clock speed and overclock stability. Also, AMD has yet to truly commit to the AM2 socket. Intel has commited to the LGA 775, which will prompt AMD to do the same.
The RAM is ok. But you are going to see mobo's (motherboards) coming out that support higher speed RAM; in fact they are already here (1066 MHz).
Also, what size is the mobo? ATX, Micro-ATX, BTX? If you want to put a next-gen GPU into the PC, you might be cramped for room. They often take two expansion slots. Plus, you want to consider case air flow and cooling, which are vital to a gaming PC. You can buy a great case, with good airflow, for $50 US.
Regarding the PSU, I have heard of Dell being quite proprietary towards what PSU's will fit and function with their set-ups.
All in all, it's not a bad rig. But if you are going to want to upgrade it and turn it into a gamer, you may want to wait and build it yourself. Consider that nVidia is going to come out with less expensive GPU's at around the same time AMD announces its lineup (we're talking good cards for $150-200). For $1500 US you can put together a nice gaming rig, that has only the things you want and nothing more. Plus, you know the quality of the parts in your system. Remember, 2 most important things: Cooling and Power Supply.
Hope this helps.