proc = AMD Phenom II X4 820
mem = 8gb
psu = 300W power supply, std atx.
MB: Supports the following processors:
AMD Phenom II X4 9xx/9xxe/8xx Quad-Core (Deneb)
AMD Phenom II X3 7xx/7xxe Triple-Core (Heka)
AMD Phenom II X2 5xx (Callisto)
AMD Athlon II X4 6xx/6xxe (Propus)
AMD Athlon II X3 4xx/4xxe (Rana)
AMD Athlon II X2 2xx/2xxe (Regor)
MB is NVIDIA GeForce 9100 based with 16GB max mem.
Net: Your processor is fine, and you do not want to upgrade processor with this MB. the only thing you need is Video.
For video: You can use a lower power card like the HD6770/HD5770 which pulls 105 watts max and see if you can squeeze under the 300 watts your PSU delivers, or you can install a nice middle power PSU like the this antec earthwatts:
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371035 and slap in any single video card from here that hits your fancy:
http://www.tomshardware.com/reviews/fastest-graphics-card-radeon-geforce,3085.html
Next upgrades... Great headphone? better mouse? SSD if you can live with a small C drive (you will generally need to reinstall software to move it to a second, data drive (i.e. the D drive)) External Disk to BACKUP your system ? New monitor ?
EDIT: No problems with cooling unless you plan to buy a 200+ watt video card. No problem in this case with any reasonable card. You will not be overclocking the processor with an HP MB, so processor heat is under control. I use an older (i.e. high wattage) GTX260 in a similar HP case without issues. If you do get excessive with the video (excellent!) then you want to pull more air out the back. A PCI slot fan is simple. Some of the HP cases I've looked at had mounting for a front fan near the bottom front panel. But I'm guessing no need for extra air.