Don't worry jlin ... those other apache processes you see are apache's child processes. Top reports 18.5% memory usage for one of them .. imagine what that would mean if they were separate processes ... you would get 100+% used only by apache, cool
To sum up, it's only one process. Memory usage is the same.
I'm also on a linode 128 and after some hours after reboot (after I played around with mc, users have accessed my web server serving php pages -- so php cached a lot --, etc etc) i have 90Mb of mem used.
I don;t think apache is the one that give the system trouble .. maybe the output of "ps aux" woul dbe of more help.
Cheers