Ideally, you want to buy it offline, as you get far more value for your money than you do on Steam.
Check out the Steam FAQ in my sig for specific packages, or look at the Steam website.
Counter-Strike, if you're new to the whole thing, I'd suggest going straight for Counter-Strike: Source, as you'll probably enjoy it more because of the graphics. It may or may not run like a bit of a dog on your system, depending on what you own.
For Counter-Strike: Source, you want to try getting hold of the Original Half-Life 2 new, as that comes with Half-Life 2, CS:S, HL2: DM.
If you just want CS:S, then you can buy it individually retail pretty cheaply, and it comes with Day of Defeat: Source. Only downside to this is that you wont own Half-Life 2 then, and wouldn't be able to use any new modifications for the Source Engine until you own Half-Life 2. But if you want to just play CS:S, then you'd be okay.
If you want classic CS 1.6, get yourself a copy of Counter-Strike 1: Anthology, can be had for <£10 retail, and contains all the other Counter-Strike variants (excluding CS:S), CS1.6, CS: Condition Zero.