Couple notes:
With NT products, MS has never done much rounding to nice pretty numbers for build numbers. NT4 was something like 1380 (well, 13xx, been so long to remember for sure) and Win2000 was 2195
However, win9x builds were figured better (.1998 for win98, .2222 for win98se, .3000 for winME).
But since XP is NT, I see no reason for MS to find a nice number until the very end, if they do. WinME RC3 was something like .2535 and then final took a jump to .3000.
Never know what they'll do.
As for the "2505a"; MS does not use letters in its build numbers. People always say stuff like that to sound more offical and confuse the heck out of people. They all said official win2000 build was 2195a, but its just 2195.
As for the XP experience; Quite partial to win2k myself. to much bloatware in XP and its all resource hogging (even turning back into standard view). The only feature I've liked so far is drag & drop cd-burning.