[Snipped out of a post by Quade]
Here's a rule of thumb. 266 bytes per post + 24 bytes per server. You can use that to calculate how many posts your machine's capable of handling.
(266 + (24 * servers)) * N (numbers of posts) / (1024 * 1024) = megs of memory used by the program
There's some other memory used too, 2-4 megs per thumbnail (high res mode a lot less in low res mode).
If you go into a group(s), have a long FTR and MRA and load up 200,000 posts from 3 servers you're going to use 67.5 megs of memory. If you have a machine with 64 megs of memory, you need to cut back on posts. If you have 128 megs, you're borderline.
If you download multiple groups simultaneusly, they'd better be small groups.