I recommend antec, corsair, seasonic, or enermax. I use ocz, which is also ok. Any standard atx power supply should fit. You may want to get a modular ps to prevent cable clutter. If you can't get any of these brands, I would post a link to your choice to get some opinions.
The official minimum recommended power supply for your (or soon to be yours) graphics card is 450W. A good 500W unit will do the job great, unless you plan to SLi or just want the extra headroom for whatever reason. As o1die mentioned, the recommended brands are Corsair, Seasonic and the newer Antec models etc. You should be good to go with any of these. Hope this helps and good luck!
But how do I figure out whether or not a power supply will work on my machine?
has your current one got a model number or anything onit, it might help us to identify what it is and therefore whether it is ATX compliant. HP and others used to have custom connections on the motherboard, I don't think they do anymore.
OK, so the power lead is probably compliant (24pin connector, but it still may not be the same as std 24 pin connectors), I was hoping to get the current psu details off the PSU itself. If that is compliant then, a) any atx will fit the space you've got, b) it'll match electrically.
I've changed power supplies several times on my hp motherboard, so don't worry about it. But cable space may be tight, so find a place inside the case to tie off the excess cables or get a cable select ps.