i think the official answer is: reaper is /3gb aware but lots and lots of plugins AREN'T. so i think the safest amount on a 32bit system is 2Gb. anything higher and you're on your own.
I'm using a 3GB switch (with 4GB installed) to good effect on Windows XP SP3 - it seems that unless I do this, I have problems getting Ivory to open while I have Superior Drummer or orchestral libraries loaded up in Kontakt... Whatever plugins I have that aren't large-address-aware don't seem to mind...
on a /3gb system, any 32-bit plugin that doesn't explicitly state that it's /3gb aware (or LAA, or LMA, or something like that) was probably just compiled as a regular dll, and will freak out when windows hands it memory over the 2gb limit. which can happen at any time (but maybe not for ages if you're not using much memory). worse still, certain versions of some plugins might be okay but older ones might not be. i just had a hunt around - there doesn't seem to be a master list anywhere of which plugins support that. i use lots of weird old plugins that are totally unsupported but otherwise stable - i'm not game to try them out on my system.
--------------------- 2007 Porsche GT3 2004 Honda S2000 Silverstone 1998 BMW M3/4 M3 SEDAN CLUB #22
Do what the name of the site in your sig says, stay calm, and go ahead and try those plugins! :-) Honestly, I have way more old plugins installed on my system than I need, and none of them have problems with 4GB of RAM in Windows XP. The only indication of anything is when loading my old Garritan Personal Orchestra, it says "not enough memory" when first loading a sample, but it goes ahead and loads it anyway and the project hums along just fine...
--------------------- 1988 turbo e30 - stolen 1987 e28is - work in progress
i'm cpu limited, not RAM limited, anyway, so it's not really an issue for me. most of my drum sounds are either from full band recordings or small sample sets from old drum machines. (rrp for, say, bfd2 is ~$800AUD. or: i can borrow an amazing vintage drum kit off a mate and get a full day in a treated rehearsal room with my portable 8 track and all my mics and stuff and it'll set me back about a hundred bucks)