At the bottom of the page, there are 6 links for the GeForc 6150, your board is one on the micro ATX ones. Look next to your northbrige for the version number, then click that link, and you'll get a page for the supported CPUs.
The board being an ECS concerns me. I had a very bad expirience.
According to HP the best processor you can get is an Athlon 64 X2 5600+ 89W max see http://h10025.www1.hp.com/ewfrf/wc/document?lc=en&dlc=en&cc=us&docname=c00906137 It came with 4GB RAM so its not worth upgrading that, so the other potential upgrade is a graphics card as you have a PCIE x16 slot anything high end would be a waste with the CPU even upgraded but you could look at a Radeon 4650 or similar.
Ok, I've already put a peppy graphics card in the x16 slot for a minor boost.... I can live with it. Thanks for your help!!
I was writing the above as azconnie replied and his link looks better than mine and suggests possible phenom processors.