Ok after some digging the best CPU that could POSSIBLY work
is the Pentium 965 EE
http://ark.intel.com/Product.aspx?id=27615
also look at this
http://en.wikipedia.org/wiki/Pentium_D
Or it looks like you can use a Pentium D (primitive dual core-two P4 cpus together on one chip essentially)
this is a guide to installing LGA775 cpus
http://www.pcstats.com/articleview.cfm?articleID=2231
I just did mine
Went from a Pentium D 945 3.4ghz to a Core2Duo E4300 1.8ghz in a Dell Optiplex 745
The C2D at 1.8ghz outperforms the 3.4ghz Pentium D
I looked up the 5100 and WILL not do Core 2 Duos
If you can find a Pentium D at a higher speed real CHEAP (no more than 30-40 USD)
possibly on Ebay used with a good Dead on Arrival (DOA) policy (30 days is good)
then go for it.
Really should consider getting new system.
I am on a TIGHT budget and picked up the Optiplex 745 for $75 USD used on Craigslist
It now has E4300,2gb ram,250gb sata II HD,Sapphire 1gb 4650 video card for total cost of $135 USD.
What I am saying is avoid putting too much into the 5100.
If you want to upgrade dont go over $75 USD total or otherwise you can start to look around for used systems.