Don't bother with the X1800 series, the only thing worth buying is the X1800GTO (when it's released later this week). The 7800GT eats the X1800XL in most benchies, although the X1800XL grants better picture quality.
The 7800GTX used to beat the X1800XT in performance, but since the X1800XT's can be found for $350-400 now, they are much more worth it.
The X1900XTX is stupidly overpriced for a 25/50 gain over the X1900XT, buy the X1900XT and save the $50-100 and then OC it yourself. You'll thank yourself.
The X1600PRO is an okay card, but pay the extra $25 and get the X1600XT or the 6800GS which is the best offer really.
Your going to have to upgrade your PSU at some later date if you ever want to upgrade, might as well do it now.