the Athlon64 3800+ is the "desktop version" of the Athlon64 FX-53 (which was the fore runner to the FX-53 and the 2nd in the FX line)
In terms of how much performance you'll see between the FX-55 and the 3800+, considering the cost differential (it's at what? $350 vs $890?) the 3800+ is far and away the better buy
this is Tom's Hardware's interactive CPU charts
http://www23.tomshardware.com/index.html?modelx=33&model1=57&model2=61&chart=26
a few notes on those charts (yeah, their already outdated)
A) when it says Athlon64 3700+, it means the Socket 754 ClawHammer version, not the Socket 939 San Diego version (the 939 is better than the 754 version)
B) It is basing the 3800+ tests on the Newcastle cored 3800+, not the current Venice cored 3800+ (the Newcastle is still avliable, both cost $366)
hope this helps...
oh, another CPU worth consideration is the Intel Pentium D 820 and 830 (the 820 is around $250 the 830 is around $320) they are the two lower level Intel dual cores (AMD's aren't near that cheap, they start at closer to $500)
those Intels won't match a 3800+ in gaming (well, they might actually, i'm not 100% sure either way on that) but they will provide you with 64-bit (the 3800+ has that) and dual core (which the 3800+ does not have)