3800 series codenamed RV670 (the cards are 3850/3870) are the new ATi cards. They are a 55nm die shrink of the R600 with 256/512 MB GDDR4 or GDDR 3 ram. They are DX10., SM 4.1 and PCI-E 2.0 compatible. They are due to be released later this month. The 3870 will beat 2900XT i think. 3850 has single slot cooler
8800GT is G92 based 65NM GPU. Comes with either 256 or 512 MB GDDR3. Single slot cooler. Faster than the old 8800GTS, though nV is bringing out a revamped 8800GTS V2 that is based on G92 that will beat the 8800GT.
The 2900GT is ATis newest GPU. Its nothing but a crippled R600 core with 240SPs, and 256MB GDDR3 with 256bit bus.
Hopefully that will bring you up to speed.