You are probably looking for a new AMD card, as stated above, similar to the AMD Radeon 6850. You can go with something like the ATI 5770 as state above to give you eyefinity for a little lower cost as well.
Sapphire Radeon 5770 with Eyefinity
ASUS Radeon 6850 with Eyefinity
Here's the problem. If you're looking to add any kind of graphics card to your existing HP desktop, be aware that it might not work because of a limited power supply. Unless you already have a high-performance gaming quality graphics card in your system that you are replacing (with a minimum 450 Watt or better power supply) or are planning to add in a new power supply as well, the standard PSU from these base systems is not enough to run additional hardware besides what they ship it out the door with (and even THEN it's questionable!)
Just to give you an example, a customer called me wondering why his
Dell Optiplex towers in his office wouldn't work anymore after he installed a brand new AMD Radeon 5890 in two of them (he wanted to start using Adobe Creative Suite on these older systems.) I pointed out to him that the included 380 Watt power supply from
Dell was 4 years old and the recommended power supply to run an AMD Radeon was 600 Watts. There just isn't enough power in the pre-built computers for additional hardware especially a performance graphics card.
Be sure before you purchase any upgraded graphics card that your system can handle it. Any graphics card that supports Eyefinity for more than 3 monitors is going to require at least one 6-pin PCI-Express power connector from your power supply and is going to require at least a 450 Watt power supply to keep it running without risking random power restarts or hardware damage.