Processors which run on AM2

depends on the BIOS.
Docs I can find point to it having an AM2 rather than AM2+ socket.
http://h20195.www2.hp.com/V2/GetPDF.aspx/c01581925.pdf tells us nforce 430 chipset
http://www.nvidia.com/page/gpu_mobo_tech_specs.html tells us nforce 430 only supports AM2

did you find that info using CPU-Z or elsewhere? can you use CPU-Z and confirm the socket?

Information:
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c01...
http://h10025.www1.hp.com/ewfrf/wc/document?docname=c00...

For the CPU:
Supports the following processors:
Athlon 64 X2 with Dual Core technology up to 5600+ (up to 89 watt)
Athlon 64 less than 4000+
Sempron less than 4000+

As for memory, you can upgrade using any DDR2 memory. You can install up to 8GB. If you have a 32-bit operating system instead of a 64-bit operating system, only just under 4GB will be usable.

As for graphics, you can put pretty much anything in, but without upgrading your power supply, I would not upgrade to anything other than a radeon 4650/4670 or radeon 5570/5670 or similar lower-powered card.

My personal recommendations are as follows, assuming the following under the headings:
You just want your computer to be faster
-Upgrade the processor to an athlon 64x2. They aren't completely gone from the market yet, albeit they are hard to find new. To notice a difference, upgrade to at least a 2.6-2.8GHz dual core. Even the most powerful athlon 64s aren't worth more than $50 these days though.
-your ram is sufficient. I doubt any performance gains from an upgrade in most cases
You're a bit of a gamer
-Same as above for the CPU.
-your ram is sufficient
-purchase and install a radeon 4670 or 5670 (both are good for their pricerange. 5670 supports dx11 and 4670 doesn't but the 4670 is still a good budget card)
You're a real gamer
-purchase/build a new computer.
HD movie watcher
-upgrade CPU as above. Everything else is sufficient.

If you want recommendations suited to your needs, let me know what you want faster and what you do with your computer. If not, then just let me know if you need anything else or have any questions.