Well the main thing is a group, and it sounds like you have that. Apart from that you need to decide which system you are going to use.
If it was 2yrs ago it's quite possible he was using D&D 3.5, so if you want to follow the same system as he was running you could go for that. Or you could go for D&D 4, which is probably simpler to learn but has been slated by some for being too MMO like by hardcore RPG fans.
Regardless if you are taking on D&D the bare minimum you will require for the group is the DM Guide, Players Handbook and a Monsters Manual. This will allow you to run the game, although you would need to create a setting/world to play on.
If you would rather avoid the hassle of creating a place to play, there are a few settings where you could pick up an additional book to get a ready made setting. Usually this will also grant you new races and/or Classes specific to the game world. Good examples are Forgotten Realms for both 3.5 and 4, and quite a few more for 3.5 including Eberron.
Theres lots of good info here for D&D
There are of course other settings, Dark Heresy for one, which is now much easier (cheaper) to get hold of thanks to Fantasy Flights reprint. It also has much more potential as they are planning on releasing new products and for a while it looked like it might have died before it began. So yeah, a 40K setting is probably always worth a look in. For Dark Heresy you need the main book only to get going, although the Inquisitors handbook wouldn't hurt either at some point.
Info on Dark Heresy
There are many others as well, as noted in the List at the top of this forum.
I would probably advise to keep it simple and first make the call of either playing in a Fantasy setting or Sci-fi setting. If you go for Fantasy I'd say grab D&D 4, just as all the new books that come out from now on will be set up with that system in mind. If you like Sci-fi, I'd go for Dark Heresy.