Welcome to Tom's Hardware Forums,
Although upgrading those can be done through HP's website here http://www.shopping.hp.com/en_US/home-office/-/products/Desktops/HP-ENVY-All-in-One/C1U69AV
, you can custom order from them, you can also do much better (price wise) by upgrading on your own.
If upgrading to a GT640, the included 300W power supply should be adequate. An upgrade to GTX650 and I will recommend a 400W or greater PSU for that.
Lets see what we can do...EVGA 02G-P4-2643-KR GeForce GT 640
for $96 with shipping, HP wants $170, $70 savings
or for the GTX650 upgradeGalaxy 65NGH8DL7AXX GeForce GTX 650
for $105 with shipping, HP doesn't offer that card. A power supply for the GTX650, I would recommend something like thisCORSAIR Builder Series CX430 430W
for $46 shipping included, HP wants $100 to upgrade to a 600W (for a GTX660 which they do offer)
The graphics cards linked are representitive samples of what is available. The NewEgg links are used for descriptive purposes only (although I do shop from them)
Hope it helps