IMPORTANT: Due to the small computer size, you can only install a small, low-profile
PCI-E card of the same approximate size of the graphics card. HP recommends that you
install a card with power consumption of 25 watts or less.
Not all low-profile cards will fit into the PCI-E slot. Some external connectors, especially
connectors located at the top of the card, may not fit properly into the back panel.
-_- not too much room there. I say the best you can do is the 5550 or 5570 GDDR3 version. There are some GDDR5 versions of the 5550 and 5570 out there, but due to the extra power draw I'd advise against it. Since 5550s are getting sparse though I would say go with this 5570
Only 25W huh O_o? Forget about that 5570 then. It has something like a 45W TDP, but when stressed with furmark (which is unrealistic for games) it pulls like 60W. Looks like you can't do any meaningful upgrade without replacing that PSU, which you may not be able to do.