List of current full-sized printers (not all-in-one) that have memory card slots:
Canon PIXMA i6000D - $179 retail
Epson Stylus Photo R300/R320 -$179/$199 retail
HP Photosmart 8150 - $199 retail
HP Photosmart 8450 - $249 retail
4x6-specific photo printers (all listed have card slots):
Epson PictureMate - $199 retail
HP Photosmart 325 - $149 retail
HP Photosmart 375 - $199 retail
Lexmark P315 - $149 retail
*Canon's 4x6-specific printers are dye-sub models that work with PictBridge cameras. (dye sub = $$$ prints)
If you're looking for a good full-sized model, I recommend Canon and Epson's offerings. HP's consumables are expensive; Canon and Epson produce superior photos for a lower cost.
As far as 4x6-specific printers go, the Epson PictureMate is the best, HANDS DOWN. It guarantees prints for $0.30 each and uses archival pigment inks so your prints will last 100 years behind UV-protected glass (plus, they're smearproof and waterproof - dry instantly out of the printer). They even guarantee that if you don't get at least 100 prints out of their print pack they'll refund the difference plus postage! HP and Lexmark's prints cost significantly more and don't last as long unless you buy their most expensive papers.