Essentially there's two different parts of the Rainbow 6 franchise: tactical and arcade. Rainbow 6, Rogue Spear and Ravenshield (and their respective expansions) are all tactical based shooters. Lockdown and Vegas are arcade shooters.
First: Lockdown blows; don't play it.
Second, if you want the true Rainbow 6 experience, you should start from the beginning and play straight up Rainbow 6. It's actually installed on my comp right now and I LANed it a few days ago with my bro. Tactical bliss, R6 and Rogue Spear be (Rogue Spear is essentially the same with somewhat upgraded graphics and weapons and all that). If you're not into old games, Ravenshield is really good aswell, though the single player is comparatively week (there's really no story at all). It's got great online if you can find a good game, and it's perhaps even more tactical based than the previous ones (wonder why it's so unpopular:rolleyes: ).
Then you have Lockdown: I repeat. Don't play it.
Vegas is an entirely different game from its predessesors. While still good, it's really more action oriented (with less brain, more twitchyness). I don't think it's even in the tactical category anymore, and if it is, it's just a flat out lie. It's got nicer graphics too, and a Conroe processor will run it quite nice. However, it really depends a lot more on graphics card; you need one that supports shader model 3.0 (that's essentially, high end cards only). And even if your card does support 3.0 and it's a low end model, it's still not going to work. You really need a fairly good card to be able to run it well (anandtech says, with high res and all the features, you pretty much NEED a 8800 card to get a good framerate). Essentially, not very well done, as the graphics aren't even as good as some other games. Perhaps post your card and we can work out how good it will be?
But as for what game you want, it really depends on what you want: tactics or action.
