[fallen1]

just wanted to add a little to what people have already said.
the primary difference between 1.6 and CS Source is the engine it runs on. 1.6 is based on the older Half-Life engine, which was designed to work on Pentium 2 and 3 computers with video cards in the 16-32 MB range. While it has seen several updates from the original engine so the minimum specs to run it at full speed with all the eye candy turned up has been bumped up slightly, but it will still run fine on something like a P3 450 with 128MB of RAM and a 64MB video card (trust me, that's pretty much what I used to run it on).
CS Source on the other hand runs on the much newer and beefier Half-Life 2 engine. This includes a much more advanced physics engine that allows for more realistic game play and interaction with the environment (in 1.6 you never have to worry about giving away your position by bumping into a tin can and having it go rolling across the floor...in source things like that are possible). Because of the advanced game and physics engines the minimum system specs are much higher. you'll need at least a P4 in the 2.0GHz and up range, a minimum of 512MB RAM, and a bare minimum 64MB graphics card (128MB if you want to have good framerates instead of just playable). pretty much any video card from the ATI Radeon 9000 series up, or Nvidia 5500 and up, should be able to run it without any problems.
there's also a big difference in game play. 1.6 has been around a lot longer, so they've had plenty of time to work out the kinks in it, while CS Source can still be a bit buggy and has some known issues they are still working on (like the hitboxes not lining up correctly when people are running). 1.6 has a faster feel to it, but CSS has a more realistic feel to it. The graphics in CSS are much nicer, including things like lighting, shadows, and reflections. 1.6 shows your recoil much more than CSS, which makes it easier to figure out where your shots are actually going.
I have both 1.6 and CSS, and personally I like 1.6 a lot more. Sure CSS looks a lot better, but it just doesn't have the same "feel" as 1.6. In the end it comes down to what your hardware can handle and what you like as far as gameplay and eye candy, but with 1.6 down around $10 there's really no reason not to get it, even if you do get CSS too. Neither one is going away anytime soon, so if your computer can handle CSS and you can afford both, there's no reason not to get them both.