Depends on what you are shooting for. I am going to assume you want the ability to have a good karaoke party for a varying audience.
I would recommend going to karaoke.com (or ebay) and looking for Supercore 2006 or Supercore 2007 on sale. That will give you 16-20 CDG disks with a very good mix of songs for young and old. A high percentage of the most-commonly-done songs are on there. And it won't break the bank.
If you then want to go to Tricerasoft.com and add 100 more songs that help you more closely fit your audience, you can burn those on about 5-6 CDG disks with a program like Powerkaraoke's Power CDG Burner. You could also do like we did for one of my dad's parties, and advertise the party long in advance and take requests for songs to have.
(For a list of songs you might download, try looking at the list for a SoundChoice Foundation 1 and 2. That will give you commonly-done songs, which may not be the ones you would pick left to yourself.)
So if you get Supercore for $90, spend $40 on software, and spend another $100 at tricerasoft.com for 100 selected karaoke songs, you spend $230 and have a very respectable home karaoke library. Certainly one good enough for a party -- the equivalent (Supercore plus a Chartbuster Essentials '60s pack) is what I started with, and we had a blast. To the point that I got hooked on karaoke completely.
I was going to say that if you had all twenty somethings you should skip Supercore, but I don't think that is true. I believe a lot of people no matter their age are going to be more comfortable singing "My Way" and "Eight Days A Week" than they will be singing "Complicated".
