for a limited horsepower, i must always recomend external reverb hardware via spdif i/o - even the cheapo t.c.electronics m300 will sound a lot better than most plugins, with the altiverb being one of the rare exceptions. i also like reverb one (which is hd only) and lexiverb (mix only), but those i cannot use with le systems.
when i have the money - which will take some time, because i just ordered a pair of genelec 8050 monitors - i will buy lexicon pcm91 and t.c.electronics m3000 reverb units. believe me, they are worth the price.
if you really need to have a good-sounding plugin reverb, why not try the "easyverb" by
www.pspaudioware.com (the plug will currently have to be run via vst/rtas adapter, but there will be a rtas version at some point later this year). it is very cheap and has surprisingly good and musical reverb algorithms; but it taxes cpu quite much, so i guess it might not be good for your sub-1ghz system.