1900xt has 16 pipes, but again (dunno how many times this has been repeated), the # of pipes are really inconsequential and mean very little (and haven't for almost 2 years now). pipelines refer to TMU's (text. mapping units), which completely leaves out the ALU's (arithmetic logic units), which are responsible for shader processing. the term "pipelines" also leaves out another important part of the architecture - vertex units (of which the xt has 8).
by comparison, the 7800 has 24 alu's, and the tmu's... well, the sort of have 24, but they share pixel duties with the 24 alu's.. the x1900 has 16 dedicated tmus and 48 dedicated alu's, which explains the the additional performance of the ati cards...
not sure if 7900s change any of this; most likely not...