It depends on what you call affordable... the prices have come down drastically on the whole of the Akai range shop around you should be able to pick something fairly decent up for less than £500, but remember you might need an external effects unit as well... if money is really tight get a SBLive, or even two and use the APS drivers on one.... on a 256MB PC this would give you 160mb sample RAM and effects 64 midi channels and 32 external midi channels.
The Emu ESI2000 looks particularly good value for the amount of features on offer. About the only major thing missing is internal resampling. Even after expanding it's RAM and adding the turbo kit and even an internal zip drive you'd still be doing very well cost wise!! It also looks easy to operate but the screen also doesn't offer any graphic waveform editing like on the bigger Emus.
Or look 2nd hand! A 2nd hand Akai S3000XL or S3200XL can't be bad. Neither could a 2nd hand Yamaha A3000 Version2.
Software wise, try the Nemesys Gigasampler and if running Logic, Emagic have the EXS24 software sampler to run with it. These are just a couple of more 'affordable' sampling options available. Cheers.