XP-M 2500s (& up) are the best on average ,many overclocked to 2.3-2.4 GHz with some going higher ,mine had an XPM 2500 o/ced to 2.5GHz

& I tested it right upto 2.63 GHz! ,though it get very hot at that speed.
You are highly unlikely to be able to buy any new skt A CPUs nowadays ,but I'm sure they are still common enough on ebay.Last time I checked (here in the UK) about 3-4mths ago XPM 2500s were going for about £25-35 ,mine sold for only £22

(not the above mentioned CPU but another XPM 2500 ,incase you're wondering my XPM is in my sons machine now

).
Btw if you do get an XPM & you have a v1.0x mbrd you will have to join 2 specific CPU pins to allow multipliers over x12.5 ,its easily done with conductive paint,a mag glass & a steady hand
However don't expect a massive boost from the upgrade, even taking into account that the XPM (Barton core version) has 512KB L2 cache (which is worth about a 5% speed boost over your XP2100) and
IF you got a good overclocker which could do say 2.5 GHz, then that's roughly a 20% speed boost over your current CPU.(Maybe a few percent more for a higher FSB too I guess).Is that worth it to you?
I guess it also depends how much you could get for your old CPU.
(Inccidently I went from an XP2400 @2.22 GHz to the XPM @2.5GHz to get the most out of my rig ,I've probably got some benchmarks somewhere if you're really interested).
Naturally if you got a CPU that could only do 2.3GHz then it would be a very small boost (at best 10%) & almost certainly not worth it.