I would recommend going with 8GB RAM and a beefier graphics card.
http://news.softpedia.com/newsImage/Nvidia-s-Upcoming-GTX-560-Ti-448-Cores-Smiles-for-the-Camera-3.png/
According to that the 550ti manages an average of around 30 FPS at 1920x1080 at high settings (Ultra is highest preset). (Hard to find benches for the 550ti in BF3 since the 550ti is a older card).
Now refresh rate on PC differs from console in the sense that the game is not locked to a certain refresh rate. They state it averages 30 FPS so I can guarantee you that it drops into the teens. Of course, you can always overclock it to squeeze out a couple more frames per second. You could also drop the graphics settings down to medium or low to ensure you stay at a playable frame rate.
But with 1 GB VRAM you are going to run into trouble with AA as well. BF3 will definitely max out 1 GB.
Depending upon what resolution and how much eye candy you want in your games is going to determine what kind of graphics card you want.
Arma 2 is a resource hog so along the same lines as what I stated for BF3.
iRacing is anything but demanding so no worries there.
If you want 1920x1080 and great looking games then I would suggest purchasing an AMD 7850.
If you are going to run a lower resolution than 1080p or don't care about eye candy then go for the 550ti.
No the Corsair PSU's are top of the line. The MSI cards are great as well. Pick up a Twin Frozr or the HAWK if you want to overclock it (better cooling, PCB, and component design). The 7870's are great overclockers. I have one pushed to 1300/1500 (1000/1200 stock clocks) at 1.3 V max temp 64 C the other is only at 1235/1400. Obviously that is a 30% increase in performance.... Picked up a minimum of 10 FPS in BF3 using only the 1300/1500 card.... I highly suggest overclocking the 7870.
Also, before I forget do not purchase a Corsair H60/80/100 for a couple months. There are issues with the impeller/ pump design that have caused malfunctioning units and an annoying "grinding" noise. It is "mainly," from what the reps have told me, specific with units in certain batches and consistent with PSU's that produce over 12.1-2V on the +12V rail. They have identified the issue but obviously they aren't going pull products off the shelves and restock everyone. Just a heads up.