Unless you want to do it the hard way (and believe me, I know what that can be like with this), certainly take a look at
Iron Heroes (if you like little to no magic, as much crunch as D&D 3e, plenty of fan support, and some third party products),
True20 (if you like low to medium magic, less crunch than D&D 3e [I'd say "rules-medium", FWIW], plenty of fan support, some third party products, and a multi-genre system as a possible plus), a less well-known and well-supported d20/OGL variant [like Conan, Black Company, Thieves' World, Grim Tales, etc.] or yeah, possibly something altogether different. . .

Er, sorry for being a bit vague towards the end there.
For Iron Heroes, you pretty much only need the single corebook, though getting Mastering Iron Heroes would be the next thing, IMO.
For True20, you basically need True20 Adventure Roleplaying (again, the single corebook), but adding the True20 Companion is something I would highly recommend.
There are a number of good options, so it comes down to requriements and taste, essentially.