Not sure if it's useful for this specifically, since I run multiples, but just some observations from my machines:
32-bit Vista Business, Intel Dual Core 2.20GHz Laptop, 4 GB RAM, ATI Mobility Radeon HD 3650 512MB
One client: medium settings, very few crashes (one or two over the span of days), happened during dock/undock, hadn't left the starting island in those sessions (was doing mining missions), occurred at terminal (Asintec Alpha). Performance slows to a crawl/rubberbanding when opening map.
Three clients: low settings, sound off, unplayable, did notice one of the clients memory footprints skyrockets when the third is started to double the usage of the others (didn't check if it was the one starting or another one, I can if that info would be useful).
Two clients: low settings, sound off (attempted to have sound on, on one client, didn't really work, cut in and out, oddest thing was that it seemed to be effected by the camera position of the client that still had the sound turned off). Occasional crashes (once every one or two days), only one client will crash, occurs at dock/undock, mining again so rarely if ever leave the one island. Occurred at Asintec Alpha, also occurs at terminal on secondary Alpha (handful of people around, but does seem more likely to occur the more others are around, crashes happen less at the secondary Alpha). Loading map causes heavy slowdown, closing map will cause clients to temporarily go "not responding".
64-bit Windows 7, Intel Quad Core 2.67 GHz, 4 GB RAM, ATI Radeon HD 4800 1024MB
Three clients: Low settings, sound off. No crashes over the span of multiple days I've used it so far. Once third client is started, see the large jump in memory footprint again. Will stabilize over long period of time when not moving (sitting still, mining) to all be equal (about 450MB for each footprint). Moving will cause footprints to go up a bit, one client will have a bit of an elevated footprint compared to the others. Docking/undocking will cause the large jump to occur again (not sure if this is every time, or just occasional, can observe more closely if that's helpful). Loading/closing of map will cause slowdown/minor pause, but clients are still responsive.
Hope that information is useful, happy to provide more/help out however I can if wanted/needed