According to the motherboard specs it's mini-ITX, so it should fit in an ATX.
And a "step up"? A 6150SE performs on the level of a Geforce 256. No, we are not talking about a 200 series Geforce, that's the Geforce 1.
6150 has 850 MT/s fill rate and uses system RAM. The 8800GT has 33,600 MT/s and 56GB/s memory bandwidth.
An X2 5400+ paired with a 8800GT can play all but the most demanding games.
As long as the PSU can handle it, of course. It's a better spot, but maybe not tenable, selling what he has and getting a SB Celeron is likely a much better avenue to pursue.
Does the HP have a standard pinout for front connectors, etc. What is being compromised to get a working a system probably isn't worth it, considering how much more his HP is worth on the used market in one piece vs parted out IMHO.
And yes, I do realize we are talking "no games" currently and "most games" in a future state with this hardware, but the 5400+ is seriously under-powered CPU. I just sold one that I had in use for years because it just couldn't justify it's power consumption based on its performance.
There are plenty of games that are going to be needing that as a minimum CPU option, never a great place to find oneself. Anything requiring a 2+ Ghz Core 2 Duo as a minimum CPU is going to be borderline with that CPU. To do all this work and then find out performance is "meh" to unusable sounds like a waste of time.
The 8800GT coupled with a better CPU (easily and cheaply aquired) is going to be cooking with gas.