Lol thats acer customer service for you the 9600gt is outdated and for the most part aside from ebay is unavailable
beyond that there are much better options for a modern probably something along the lines of a 5770 (6770)at most.. id also start ocing your cpu which is weak or look to upgrade to something along the lines of a p2x4 955
You will be limited by the PSU in your system, unless you decide to upgrade it also. If your PSU is less than 250W, then you would not be able to install anything strong enough to be worth the upgrade. If it is 250W to 300W, you should be able to run a HD5570 or HD6570. If it is 350W, a HD5670 or HD6670 should be safe. Anything stronger would be too bottlenecked by your CPU to be worthwhile.
Agreed with the above person, 6670 is probably the top card you will be able to get... Maybe with the 450w you could get the 5770 or 6770, which are decent mid ranged cards. I would personally just save your money for an upgrade, but if you are dead set on just getting a new graphics card, and you have the 450w, I would say 6770 is your best bet. But like jtt283 said, it might be bottlenecked by your cpu
bottom line dont listen to Acer customer service, they are idiots... likely a 5670 (6670) is the type of video care you are looking for if your psu can handle it although I have a backup rig with a 240 regor oc'ed to 3.6ghz and a 5770 and it does quite well at 720p and there is little bottleneck other then the fact that you will be held back in games that need a healthy quad but that is no gpu bottleneck