GTA IV apparently runs on Euphoria engine. Now other games that run that same engine and the most recent I can think of Crysis which needs
CPU: Intel P4 2Ghz or AMD equivalent (3GHz dual core reccomended)
RAM: 512MB (1 GB rec)
Video Card: 256mb
HDD Space: 16GB
OS: Microsoft Windows
Sound Card: DirectX 9.0c Compatible
I would guess that you would need a really good graphics card but at least a 256mb, at least 2gig ram and about 20gig hdd. Obviously as all games its made for windows and I dont know how easy it would be to run it in Wine in Ubuntu.
SO.. It would be safe to say that the above stats would be required by to play the game.
As soon as we find the real requirements from Rockstar we will post them here.
UPDATE:
Please not this is only a guess if we look at current games. As soon as Rockstar realeases the official stats we will post them. But you could use these, if so do that at your own risk.

New Nano Suites available at www.crymod.com
Complete Gallery, 35 pictures, available at PCGH
An interview conducted by PC Games Hardware with Bernd Diemer of the Crysis Warhead developers team has confirmed that Crysis Warhead’s graphics options will be equally as splendid in DX9 and DX10
FULL STORY
Author: Reme Le Hane - Cyburbia Computers

A new mod for Crysis, an already extremely popular game is going to give it an great boost for those who can afford to see it. This new mod in production by Havoksage, a member of crymod.com.
This mod is going to improve on the particle effects that already make this game so amazing to look at.
Hers is what this mod currently offers:
• Fires can start if an explosion is near foliage or buildings, not dynamic at the moment.
• Improved wood pieces and splinters, with additional 3d models.
• Much longer lasting smoke.
• Small landslides when cliffs are shot.
• Soil and dirt is kicked up from explosions.
• Additional dust on tree break.
• Additional physicalized blood particles
Author: Reme Le Hane - Cyburbia Computers
You already have the ability to pimp out Crysis with great mods like Mster Config. One mod currently in the works is a texture upgrade from Rygel. Rygel who is famous for his Quake and Doom textures. Mster Config with Rygel’s texture mod is a very powerful way to tune the graphics in Crysis.
Here are a few screen shots of what is to come from this excellent combo
I cannot wait for the finished release. For more images you can visit PCGH. They have the other 10 images.
Author: Reme Le Hane - Cyburbia Computers