Well, if the server utility and OS fully utilize multiprocessors then the Dual P3 800's should be better... You're just going to have to try it and see. Remember that serving BF requires as much fast RAM as possible. If the P3 only say 512mb PC133 and the athlon has 512mb DDR 2100 or better, then I'd try the DDR board. I'd also buy another stick of RAM. I probably wouldn't bother trying to serve without 768+ RAM. I won't even play BF without at least 512mb RAM (Mines 1gig DDR 3000).
Just give it a shot and see how you do. Try making a 18 player server first. If that works well, push it to 24, then 32. I doubt it will handle over 32 players, even with 1 gig of RAM. If you at least have 512mb then I'd think the athlon should be able to handle 24 players and maybe 32 without too much lag.
all I can say at this point is give it a whirl!
