GTA IV runs slow, but I have a powerful rig?

My specs are:

8GB System RAM
I play at 1920x1080 resolution!
AMD Quad-Core 2.4GHz rated at:
ATi Radeon HD 6550D (D is for Desktop graphics, f*ck laptops.)
1TB HDD
Genuine Windows 7 Home Premium, 64 bit.

I thought of downgrading to Windows XP. Should I? (Pirated) and please realize in GTA IV, under "Display" my available video ram is 489... wow. WTF, I haz 4GB's that is must not be interpreting. I so hope GTA 5 Isn't this much of a disaster!

Um, anything else I should add? According to "Canyourunit.com" I'm BEAT recommended, but I only get 1-10 FPS. Please help me?

Sorry to break it to you, but, no you don't (have a powerful rig)...at least by GTA standards.

Firstly; All the GTA series are CPU clockspeed sensitive. 3.4GHz I would consider "entry level" for the most part, and anything above this would definitely be better.

Secondly; Your graphics card....basically NO, unless you're turning down the graphics options (esp. draw distance) way, way down and using a low screen resolution. Personally I'd place as much faith in "canyourunit" as I would in used car salesmen. You can try YouGamers but this is also going to be very optimistic

As divide said, turn down your settings/resolution.
That isnt a "powerful" rig,
P.S 1920x1080 isnt necessary for that specs.

On top of what dbz said, also, don't mention anything about pirating OSes again or you can kindly GTFO.