eaccelerator on itself is mainly used for pre-compiled php. But if you install it with the "shared-memory" option, you can let it work like memcache too.
Only downside is, as far as I know, it only works on the local server. Whereas memcache allows you to offload all cache to another server.
Upside is, faster PHP-interpreting and auto file/memory caching rotation.
IPB is using partial caching. Meaning it won't cache direct results, but parsed, sorted and prepared data. Something I prefer and make use off myself too.
So it doesn't really matter much what caching you use, if you know the advantage of the one over the other.
I'm pretty sure IPB does not support MULTIPLE caches though... otherwise you could use a combo of eaccelerator and memcache.