first...
ignore all "urban myths" about how bad "winmodems" are etc....EXCEPT you need a modem which also might run under linux, whatever.
I have/had several win(soft) modems..and NONE of them uses higher CPU power than say....0.5 % downloading.....its TOTALLY negelectible and imho no argument for going with an hardware modem instead. (Taskinfo2000 is a very good program showing all threads/tasks and how much CPU time they're taking)
We at work use Lucent portmasters, and in general it's said that modems (if winmodems) based on the Lucent Chipset are best. I cannot confirm that (i did extensive testiing !)....i also have a creative modemblaster di5655 which *for my situation* gives better results than the lucent modem. (Problem is frequent retraining and falling back to lower speeds without going up to higher speed again).
But...this all depends HEAVILY on your specific line conditions, the equipment your ISP uses etc...and this can vary from line to line/person to person.
Right now i use the creative modemblaster (its a ESS chipset based, the cheap one from newegg)....and i get constant good speeds (48000 downspeed, 16800 up) with my bad lines...and no backfalling at like 28800 bps as with the lucent winmodem.
I also heard that Zoom modems should be very good....but honestly i'd never spend like $80, $90 for a freakin' modem......i think the times for modems are over already anyway and i am using my modem(s) only as temporary solution 'til wireless/dsl is available for me.
It MAY be worth also to get a few of the really cheap winmodems (about $15) and try out which one of these works best....but the problem may be that Joe Average may never see the "real" speeds but only the speed the modem connects to the ISP. If the modem falls back to 28k after a few minutes connect windows still shows that you're connected like at 48000 (or 50000 etc...)....except your ISP (somehow) provides you with the real data (up/downspeeds) from the portmasters in real time. (Almost never).
I have the advantage to see the real speeds because i work at an ISP which makes it easier to determine which modem is better in REAL life...