It's an OEM computer so you're not really going to be able to do much. Most of the CPU's that you can use in that motherboard are kind of old and out dated now. I would save the money and start saving up for a new computer.
Maybe a bit late for the OP, yet there's many of these former business/corporate PC's sold off lease for as little as $69, what I paid for my HP dc5800 SFF PC. Shipped with a Intel Core 2 Duo E8400, and though I could had went with a Core 2 Quad 9650, didn't because of the low profile 1GB GDDR5 GPU that came out of my XPS 8700 when new, just had to install a low profile bracket, it's the same card that Dell used on many PC's of that era, and also has a UEFI BIOS. Bought another like it for $36 for my Optiplex 780 DT edition, which in size not much larger, which runs a Intel Core 2 Quad 9650, which was installed already.
Had I been running onboard graphics, would had pulled the trigger on at least a Q9450, though the Q9650 runs at 3.0GHz, both has a 12MB L2 cache (so does the popular Q9550), and can hang with many off the shelf PC's of today, especially the sub-$500 market.
Here's the Speccy snapshot of the upgrade I performed, and yes, has ran for nearly 23 hours non-stop.
If only there was a 300W PSU option for the PC, wouldn't hesitate to pull the trigger on a Q9650/9550/9450. Even the lesser of the three is stout w/out Turbo Boost, which none of these has. Being an OEM PC, means no overclocking, but at this age, one should care for their PC than look to run it at very high temps.
My point of this post are that these PC's are upgradable, including GPU & SSD. :-)