Sorry, I finally have to chime in!
Please look up Husqvarna's web site (lot's of info)...you can search for many manuals. The Husqvarna 51 does come up in the search. I'd attach it, but I haven't figured that out yet. Anyway, as you can see in the manual the 51 is a 49cc saw.
Now Snal, you said you had 24"+ dia. trees and want to cut through once. That's gonna need a bigggg bar! A 24" bar is pretty big. Bigger than that is wicked big!
Shasta said he has a 385xp. That's a wicked large pro saw and with power comes weight,84.7cc! (I'm jealous though /forums/images/graemlins/laugh.gif). Also it comes with a price...way more than $500 on average for sure!
Now, a 24" bar is on the end of the range for a 359, and a 357xp (xp's run at a higher rpm...fast!). I'd suggest bigger if you really need a 24" bar. The oiler system will be stretched in capacity to deal with that much chain in a hard wood tree (oak right?).
The newer 570, 67.9cc, might be a little more up to the task, yet still smaller than the 385xp and cheaper (still might be looking over 6 bills unless you find a deal on-line). Also, you might even be able to find a 372xp. This was disconntinued in the US and recently might be available again in small quantities. It is touted as Husqvarna's most popular pro saws and the 570 hasn't seemed to gain ground yet. Either of these will run a 24" bar very well. A word of caution from the safety police...these saws are much bigger than a homeowner saw and require proper handling and PPE!
Last thought...as most folks on this great sight mention, dealer support is everything. Like you guys say for tractors goes with saws too.
Happy shopping and be safe!
boatermark