I'm really clear on what exactly you want to do. If you want to keep the instrumentals part of the original songs, and add your own vocals, then yeah, you might as well get a kareoke machine.
If, as I kind of suspect, you're saying you want to get into REmixing (putting your own instrumental accompaniment to the vocals, and changing the order of the vocals around a bit), that's a different story. While the methods you referred to do a decent job at removing a vocal, I don't know of any for isolating a vocal in a commercial track. If you want to get into remixing, I recommend looking for remix contests online - I THINK acidplanet.com has them monthly, and if you root around, you'll always find a few going on - this forum recently had a thread about a Thrice remix contest, which is probably still going on. In summary, there are lots of better places for source material than commercial CDs.
As to software, I see there as being two good starting points:
1) Fruity Loops Studio - this is very rhthmic-oriented - that is to say, its best for making original beats, and repetitive synth lines. Some people around here look down on it as being overly simplistic, but there are also some serious musicians here who use it (particularly in hip-hop and related genres). One of its benefits is that you can run it slaved to some of the other, more full-featured (but expensive and complex) sequencers. I record almost everything in Sonar Producer Edition (~$400), BUT I make all my drum tracks with Fruity Loops. It's not limited to beats, it just specialized in them - you'll be able to import vocal lines, etc. if you go with this. The website is
http://www.flstudio.com - there are a number of versions, that range in price from $50 to $300 - to get the best audio support, you'd have to go with at least the $150 version. That does include free upgrades for life, though - its really a good deal. I'm unclear on how much you can do with audio with the $50 version - you should probably investigate this before making a decision.
2) ACID - I know a lot less about this, but they're the ones who host the monthly remix contests I mentioned above, and I think its designed for remixing. Have a look around their site:
http://www.acidplanet.com/tools/?p=acid&T=3330
It looks like their basic version is $70.
As far as I'm concerned, which one you go with should be based on where you might want to go with this. If all you want out of this is a small remixing hobby, and you don't really intend get the whole scope of home recording/music production, go with acid - its designed for what you want to do. If you're testing the waters to get into music more seriously, and you want to start developing a specialty (making beats), go with Fruity Loops - it will remain useful even if your studio and interests grow.
After writing all that, I hope I guessed your meaning correctly
