Firstly go PC, the console route, whilst engaging, doesn't
really have a decent sim on any format (they've got cracking "psuedo-sims" which are alot of fun, but if you're after as real as you can get, forget it it, there's no competition).
As for recommended titles, you
must get hold of GTR2 (which is a hardcore sim of sports car racing) and, if you like Touring Cars, Race '07 (with the GTR Evolution Add-On) is well worth a punt too. These sims are from SimBin racing who've in many ways taken over from Papyrus as the current kings of PC motorsport,
you can learn more about their stuff here.
Also worth a plug is
Rfactor which is more of a sim racing system than a complete package. What is excellent about it is that the wider sim-racing community have got together to make many, many, many modifications, all pretty much free, and some that are truely incredible. The F1 1979 Mod is a work of art, utterly incredible and insanely detailed. It's also very challenging,
here's an embarrassing video of me trying to hustle an Ensign round Kyalami in it (on the most realistic settings) fast enough to qualify for the real 1979 Grand Prix. And to think . . . people let me near
real racing cars . . .
Of course, the grandaddy of all the modern
real sims is
Grand Prix Legends, which I still play alot, it's excellent modding community has kept it up to date in terms of graphics and physics. For visceral raw thrills it's still the best sim out there I think, and you can pick it up for a fiver off eBay. And let's face it 60s F1 cars are way cool . . .
If ovals (specifically NASCAR) is your thing then the only option is
NASCAR Racing 2003 from Papyrus (a legendary Sim-House). It's still the best out and out sim of racing on a super-speedway that exists, and once again the Modding community is very active, so updating it to 2009 is no problem at all. There's also an excellent on-line racing community for this Sim and Championships are rewarding and regular.
I'm going to plug
Dirt Track Racing 2 here aswell. Nobody's ever heard of it (it's made by a now defunct Australian publishing house "Ratbag") and is a sim of saloon and modified racing on Dirt Ovals. It's fantastic, has low systems requirements, and I can play for hours on it. I'm mainly attracted to it because my real-life racing has usually revolved around 1/4 and 1/2 mile ovals on dirt and for side-by side, gritted teeth, racing it's (in my humble opinion) the best there is.
And finally there's the new behemoth that is
iRacing which is a subscription based online sim where you'll race others from around the world in various classes and vehicles. It's a phenomenal acheivement and there's always going to be someone around to race against (Including a number of professional drivers you'll probably have heard of). It's currently the new leader in terms of race sims and it's very, very, very well done. I've driven a Legend in real life and the experience corresponds pretty much exactly (aside from the obvious, unsimmable, factors) with iRacing's version of it. It's stunning, but the subscription, plus extra costs for add on cars and tracks, can soon add up if you're not playing it regularly. But then again I'm Scottish and mean, so that might just be genetic ;).
Anyway, I hope that's been of help

.