I remember thhat I used to save up to buy oblivion games and then buy guidees to help me clock it. Oblivion is endless and lots fun. But scary XD. Get it if your sure it won't give you nightmares =P
At the moment, the original game comes to about 5gb of data. Individually, just the mods I'm using exceed 10gb. It can be very fun to find mods that improve, not only graphics, but gameplay as well.
As much as I loved bioshock and mass effect, you will get WAY more playtime out of oblivion: elder scrolls especially if you liked fallout 3 considering both are made by bethesda. Both are packed to the brim with rpg goodness and really bioshock and mass effect are light on the rpg mechanics.
You should also keep an eye on Fallout: New Vegas coming later this year.
oblivion has a ton of mods and you can customize your game experience further. Mds out the wazzu. I suggest ME2 if you want an rpg with guns as bioshock feels the least like an rpg to me.
Oblivion FTW! With mods you'll never run out of things to do. I've spent over 400 hours playing the game and I'm not sure I visited everywhere yet. I've had to restart characters (for various reasons) but it's still an amazing game.
I've installed mods that add castles, companions and a mod that adds weather and better graphics.
www.tesnexus.com<--- great database for oblivion mods