the 460W should be ok, but it would be hitting its max and if it is a low quality power supply, the system may reboot or crash when it reaches its max.

the 460 gtx looks like a brick and takes up 2 pci slots (fits in 1 slot but hides another slot due to its size) but should almost be the same length as the 4650. The only problem I see with getting a new video card is overheating, which might be happening now with your computer.

premade computers from HP,dell,etc are usually packed into a small case to save on costs. this makes installing new hardware or cleaning difficult. Also these systems are prone to overheating since not much heat leaves the case and will cause system failures and or slow computer if the processor has to underclock itself if it has that feature.