Are you actually hitting the limit in practice? So you have 50+ concurrent requests at once? Is this a "real business"? If so you shouldn't be so afraid of putting in the time to setup nginx or something else... you'll literally _have_to_ to different media serving and load balancing if you plan to grow whatever_this_is.
As far as actual RAM usage, you'd probably save some space and lose very little (depending on what your app does) on a 32-bit OS. Sadly, Slicehost only offers 64-bit... you might check out Linode or one of their other competitors. I'd only suggest this option if you're really broke, though. You'll still have to do the work mentioned above on Linode eventually... the 64->32 would only be a small stopgap.
I don't do any PHP anymore, but I think thread-safeness largely depends on what external C libraries you use, and generally isn't guaranteed.