Would also like to add that I was thinking of upgrading to a 750w Corsair with another R9 270x in crossfire, would that resolve the issue possibly?
I think it will not affect too much, crossfire program might be useless for GTA4, unless you are trying to make multiple monitors view.
R9 270 is good GPU card,it can run most recent high graphic games smoothly, but somehow, GTA 4 can not detect latest GPUs dedicated memory ( Sometimes, even if you have 4GB dedicated memory in your GPU, GTA 4 will say it only 1Gb)
So like Pokedude said, you still need to put commandline.txt in your game folder then it will fix it.
Another suggestion :
1. You might need to defrag your GTA4 game folder by using Contig.exe (http://download.sysinternals.com/files/Contig.zip)
Some players fixed FPS problem after using it, and it gives you better performance.
How to use contig:
what is Contig ?
Contig is a single-file defragmenter that attempts to make files contiguous on disk. Its perfect for quickly optimizing files that are continuously becoming fragmented, or that you want to ensure are in as few fragments as possible. It can make your games smoother and get better performance ( works also for GTA 4 )
Using Contig :
1. Download Contig.exe (http://download.sysinternals.com/files/Contig.zip)
2. Extract and put contig.exe in your GTA IV game folder
3. Right click and send it to the desktop
4. Back to your desktop, right click on contig shorcut and click properties
5. Add " -s" ( space -s ) without quotes at the end of target box ( as shown on the picture )
(https://fbcdn-sphotos-c-a.akamaihd.net/hphotos-ak-xpa1/v/t1.0-9/10929987_580227052121420_5239135655345189652_n.jpg?oh=07f32fbee5c5c9472b908b8a2c2ea663&oe=5568A216&__gda__=1431929006_dfafa101ee02466edb68c0353382ba7f)
6. Click ok to apply
7. Double click your contig shortcut and let the program defrags your game until it finished.
8. Run your game.
2. You can download this modded files and replace the original files with these, it will increase 10-15 FPS of your game
Download files here : Boost FPS (https://www.mediafire.com/?z8bwqlexgblbcq0)