Chris is right - running multiple webserver apps on a single server can cause problems (trust me - I know from experience what one crappy server trying to do everything is like >_<)
I personally run my own home server splitting services between a PC and an Apple XServe and its much faster than it was when I ran everything on just the PC (P-4, 256 MB RAM). If you can afford it, its a better bet, but it also depends on what you need it for and what kind of server your buying.
If you need the server for more high-end needs (website hosting, for example) then a more expensive server (or multiple servers for intense use) would do the trick. For something like a personal FTP server, you could go for a more low end machine (like my PC, the XServe is for webhosting).