For the drivers, go
HERE...it's fairly obvious from that point but there are good FAQ's and essentially, you just check you're up to date, download the latest driver pack if you're not and run the .exe file.
The sliders I'm talking about will be in the game itself, in the options section, usually under performance of graphics or some ething similar....rather than trying to run the game at a really big size, set it down to 800x600 for example, and see what it's like.
You can also turn down a lot of things like detail in shadow effects and so on in some games..... they may not look quite as pretty , but should run a lot smoother
Trial and error really.....if you then find you cant play your fave game smoothly or you have to compromise on visual quality too much, upgrade the card.
Don't go for the latest bleeding edge card though....something 2 or three models down the line up from most manufacturers will still be pretty damn quick and probably only 6 months old in design, and you'll pay half or less of what the bleeding edge 'must have' card will cost you.