i think common sense will be your best method for figuring this issue out. As with all new releases, the game will require a system with similar spec's to other current new triple A titles. I would bet money that since it's a large world, with variable draw distance, that Vram will play a large part in performance, as will a decent Quad core CPU(Also because of the open world with Many moving pedestrian's, vehicles, etc..). I would guess that an AMD-260/270 or Nvidia-750/760 would be smart BASELINE choices for GPU's, as long as they have 2Gb's of Vram (These cards will run @ 1080, but I wouldn't expect Great frame rates, or High settings).... Based on the previous versions of GTA, Mainly the 4th, 1Gb would be pressing Your luck. I would go with a MINIMUM of 2Gb's(BUT that doesn't nmean it's My opinion that You need 4Gb's of Vram, I just wouldn't go with less personally). If You want to be on the safer side, AMD-270x/280 and Up......Nvidia-760ti/770/ etc and up(Or comparable performing Card's). At the end of the Day, the PS3, and Xbox1 run this game, So You won't NEED a titan, or 4Gb's of Vram to run it @ playable frames. Even if They re-do the ENTIRE game for PC release (which they Won't) They're not gonna expect people to have Mega gaming machines to run they're game, the point is to sell to as many customers as possible, and developers would be insane to make it obligatory to upgrade JUST to run they're new release.
IMO, a 5800 anything is pushing it, ESPECIALLY considering that the game will cost $50-$65 USD, and that's a Lot of $$ to spend when You THINK your GPU MIGHT be fast enough. Tell him/her to upgrade, unless of course He/She doesn't mind finding out @ release if the GPU is or Isn't up to running the game, I would bet on the latter over the former though. If it has 2Gb's of Vram, it might just be able to do it, @ 1366x768 I would bet that it COULD run it, even if the settings, and draw distance need to be turned down, that's basic Laptop resolution.
****EDIT**** Another smart idea, would be to pick a game that is out now, that is similar, and see if His/Her GPU can manage THAT title. I would suggest Dead Rising 3, although it has More on screen @ once, i.e. hundreds of zombies, the action is slower, and the zombies are repetitive, and replicated after 8-10 unique types, So in the end it is a decent way of measuring up capabilities. Also, WatchDogs would be a decent measuring method. I'm sure there are other's that will provide a similar result, but these are a couple I could think of.