On my Vista PC, Radarbox uses:
Private = 92MB
Shareable = 10MB
Total = 102MB
Biggest memory hog on my PC is Internet Explorer at 270MB. I have a total of 2GB of memory, and regularly run IE, RB, Planeplotter, FD6, Shipplotter, Outlook, Digiguide, etc. with no problems.
CPU load for Radarbox averages 15% (on a 2.8Ghz processor), using 10-12 threads.
Are you running an anti-virus software, and have you excluded RB folders from its scans?