I just moved my PVR into an epox mini case, it's like a Shuttle XPC, just not made by Shuttle. It's an Athlon XP 2500+, and the temp right now is 54C and it's been rising, so I don't know where it will top out at. That temp is a little high compared to my last case, which was usually in the 40s, but is this too hot?

I would start worrying at around 55-56, and definitely shut down at 60. These are on the high end of course, and I wouldn't run it 24/7 at over 50.

the xps run hot and can handle the heat, anything under 75 is fine

60 is probably the max I would go. My xp 3200+ runs at 45c when it idles and only gets a little bit hotter under lots of stress.

75c seems more like a GPU temp, if your CPU is running at 75 and you are getting any crashes, I would invest in some new cooling solutions, or make sure your thermal paste is applied correctly.

I removed the thermal grease that came on the heatsink and put on some Antec grease I had laying around and it seems to have reduced it a bit, so I'll keep an eye on it but at least its down now.

Everything in this case is crammed though, there's virtually no room, so the lack of airflow certainly doesn't help either

I've worked with a fair amount xps/socket as, some of them just flat out run hot. Most of them ran between 50-70ish c temps. I wouldn't worry about it being in the 50s-60s at all.

Yeah, XPs just run really hot. I had a 1800+ that usually idled at 55C and went up to 65C loaded with the stock heatsink that AMD provided with the system.

They arn't the most power efficient processors AMD has ever made though, so as long as it doesn't crash from heat, i wouldn't worrry about it.