according to the specs:
http://en.wikipedia.org/wiki/ASUS_Eee_PC
the Eec comes with 512MB or 1GB
of course 1GB is better but i think even with the 512MB version you can load pretty large sample sets much bigger than 20MB.
since LinuxSampler can run without GUI, in case you are short of memory you could boot the Eee in console mode and then just
feed LinuxSampler with an LSCP script which sets up audio,midi interfaces and preloads the samples.
You can even save ram using the ON DEMAND LSCP commands so that a MIDI program change loads the sample on demand thus
conserving RAM. That way cou can remote control LinuxSampler only using MIDI commands without using a memory hungry GUI.
And who where the ones that said that separating engine and GUI frontend was a bad idea ?
As for audio,midi playback, not sure how good the internal audio interface is. (both audio quality and latency wise) but you can always
resort to an external USB audio/midi interface, but sincerely I don't know which one to recommend so just ask around before buying something.
Looking forward to hear about your experiences with the Eee, it is an interesting machine and I am sure some musicians will put it at a good use
since it is small, rugged and stable as it runs Linux
good luck !