Heho,
I use an iMac with 16gb of ram and it works really good! I wouldn't look to get too expansive (haha says the Mac user xD) as the more expansive pcs have other features you really don't need (like really good graphics). Just a basic pc with 16gb of ram with a decent intel i5 or i7 (at least dual-core) should do the thing!
Really big sessions will always need freezing (in reaper it is called like this; it is making a wav file with all the effects on replacing your normal file so your effects don't need to work in realtime while storing the setting so you can "unfreeze" it anytime) so the rest I think doesn't matter much ^^
cheers, Niki