Flatbed slide scanner attachments generally suck. Even the Epson Perfection 4870, which costs around US$450, is no match for a dedicated film scanner.
With film scanners, you will need to decide if you want "Digital ICE" (or FARE, as Canon calls it), which is a technology that uses an IR light source to accurately remove most dust and scratches (it doesn't work on true B&W film, and can be unreliable on some Kodachrome).
If your slides aren't terribly dusty, and you don't mind despotting in Photoshop, then the 3200DPI Minolta Scan Dual IV for US$300 would suit your needs well. If you need something a little better, a 4000DPI Nikon Coolscan V with ICE for US$600 would be a good choice.
Beyond that, there is a 5400DPI Minolta scanner (which probably outresolves most film anyway, and I've seen numerous reports of low build quality), and a Nikon Coolscan 5000 (also a 4000DPI scanner, but significantly faster than the V and takes more accessories that most of the rest of us will probably never need).
Edit: And since places like Newegg generally have a very limited selection of scanners, you'll probably want to buy from
B&H or
Adorama (both are reputable dealers - I've ordered several times from the former, and have been satisfied).
Edit 2: Stay far, far away from Pacific Image scanners; not only is the build quality rather crappy, but I have heard reports of film being scratched by the poorly-designed scanners.
Here is a list of scanners to get you started.