Intrusion Detection comes in two forms: Host-based and network-based. Each protects against things that the other cannot. While I would not want my server running the NIDS, I do run HIDS (Tripwire) on all of my servers.
All NIDS are network sniffers. But for my money, I'd run Ethereal for sniffing, and MRTG for statistical analysis. If I really want graphics on the sniffer, I use Etherape. All open-source software (read "FREE"

and I haven't used anything that I thought was more effective, although some of the Network General stuff has been more user friendly.
Take a look out on Source Forge,
before sinking your entire budget into something that may not provide you what you want/need.
Not that I don't use commercial software, I do, but I always use a free version first, to figure out what is good and bad about a given class of application before spending money on something that doesn't do what I was expecting.
If you are afraid of Linux, try Knoppix,
It will allow you to run Linux on one of your systems without modifying it in any way. It is a bootable CD and has most (all???) of these apps installed.
pansophic