my ipb version 2.3.2 and have 3700 members and 30,838 posts. database size is 11mb
max. 25 members online at a time. and i am using shared host.

i not have any problem with host. but some time i get ipb error like "server can not handle your request".
server have php ver. 4.4.7 and mysql ver. 4.1.22

so i need to buy VPS or server for my host?

No, you shouldn't need a VPS or dedicated server for that. I have a board with 20,000+ members and 120,000+ posts and have never had a problem with my host. If you're interested, I could refer you to probably one of the best hosts on the web. (in my opinion)