According to the link below, desktops with Q chipsets cannot use 5xxx Xeons:
http://www.delidded.com/lga-771-to-775-adapter/2/
This group of Q chipset desktops includes the HP DC 5800 (and a whole bunch of other HPs). Desktops like the Optiplex 360/380 (with G chipsets), on the other hand, can use 5xxx Xeons (at least the C0 stepping ones).
However, one concern I have with the Dell Optiplex 360/380 is the weaker PSU (only 14 amps on the 12v rail compared to the 19 amps combined (11.5 and 7.5) on the two 12v rails for the DC5800).
With that said, the Dell Optiplex 380 does take DDR3 ram, has built in Wifi and the price for the slim desktop version (from my limited ebay exposure) doesn't appear to be that much more expensive compared the DC5800.
The DC5800 does have more RAM slots (4 vs. 2) and more SATA ports (4 vs. 2) though. It is also slightly smaller at 12.4 liters compared to the slim desktop version (16 liters). (NOTE: A 10 liter SFF version of the Dell does exist, but at the moment, it appears to be more rare)
So it is a tough decision to make.
If I go with the DC5800, that leaves me with the just the 3xxx series If I want to use a Xeon.
So which 3xxx Xeon do I use for this low budget project?
With the HP DC5800 being natively compatible with the Q6700, Q8200, Q8300, Q9300,Q9400,Q9650 (source:
http://www8.hp.com/h20195/v2/GetDocument.aspx?docname=c04290830 ) I figure it is probably best to attempt a 3xxx Xeon with the same clockspeed and cache as the quad cores listed. Based on my budget goals I'm thinking of using a x3323 Xeon (2.5 Ghz with 6MB cache and currently $18 "buy it now" on ebay, free shipping). This (plus $2 adapter and modification of socket) gives me the performance of the $40 Q9300 at a 15 watt lower TDP. Not exactly a huge savings overall, but at this budget level and small form factor I will happily take the $20 and 15 watt reduction. This provided the modification is easy enough and the cpu works as intended.