it is getting harder and harder to find these cards at a reasonable price these days, and many being sold are not working properly, so the best advice I can give you is to make sure who over your buy it from is a reputable seller and offers some kind of warranty on the card.
You maybe able to buy the card from Dell but if will be expensive.
Ebay maybe you best choice but choose the sellers carefully.
Also if this is for a E1705/9400 laptop or a M1710 laptop you have several choices of cards
x1400 - Not a gaming card
7900GS or the FX1500 (same card)
7900GTX or FX2500 (same Card)
7950GTX or FX3500 (same Card)
and possible the FX1600.