I just purchased your marketpress plugin and network activated it on my wordpress 3.0.5 multisite install with about 25 sites. My hosting provider is hostgator, I have a VPS with 1.13Ghz cpu, 768MB memory.
I usually only see about 300MB of memory used and between .1% and 12% cpu usage. as soon as I activate marketpress both start to climb until they max out and crash the site. I have to restart httpd to get it back. It takes about 5minutes for it to go critical