This question is asked so often that I have this text file saved on my hard disk and ready:
50 posts--TMF Novice
150--TMF Regular
300--TMF Expert
600--TMF Master (although I have always contended that it should be TMF Mistress for women members)
1,000--First Level Red Feather
1,250--Second Level Red Feather
1,500--Third Level Red Feather
1,750--Fourth Level Red Feather
The four-levels-per-thousand with a different color continues on upward until 9,750 for Fourth Level White Feather, with the colors as follows:
1,000--Red Feather
2,000--Orange Feather
3,000--Yellow Feather
4,000--Green Feather
5,000--Blue Feather
6,000--Indigo Feather
7,000--Violet Feather
8,000--Black Feather
9,000--White Feather
These levels have been reached so far at 10,000 and above (where the "First, Second, Third, Fourth" are no longer used):
10,000 Level of Cherry Feather
11,000 Level of Tangerine Feather
12,000 Level of Lemon Feather
13,000 Level of Lime Feather
14,000 Level of Blueberry Feather
15,000 Level of Raspberry Feather
16,000 Level of Grape Feather
17,000 Level of Blackberry Feather
18,000 Level of Diamond Feather
19,000 Level of Double Diamond Feather
20,000 Level of Triple Diamond Feather
21,000 Level of Quadruple Diamond Feather
22,000 Level of Quintuple Diamond Feather
I have a two year head start on you (registered April, 2001) and you're only a hundred or so posts behind me! 
