Help! Need advice on upgrading my computer parts! 

These are both my monitor and my PC 

I need a new monitor, graphic card, and a powersupply

my graphic card is a 6540 Radeon 

My powersupply only goes up to 150-200 watts

My monitor resoultion is 1600 X 900

I was thinking of buying these parts

I wanna play most modern games to medium, high, or ultra setting on 1080p 

My budget is 400$

For your budget and cureent CPU those parts look fine. AMD CPUs work well with AMD CPUs. Corsair power supplies are great, 600 watts is plenty. The monitor look good, I like Asus Monitors (I use two) 21.5 inch is a bit small for me but that's personal preference. AMD is known for running hot so keep an eye on your thermals. Those parts will run most anything at High to ultra settings. Good luck and I hope you enjoy what ever parts you pick.

I'm assuming you mean AMD GPUs work well with AMD CPUS. Doesn't matter. nVidia or AMD or Intel. Pretty much the same. You don't have to pair AMD/AMD or Intel/nVidia. Doesn't really matter though. I don't see much reason to buy a nVidia GPU right now unless you are crazy about power consumption. They pretty much dominate in every area.

AMD products don't really run much hotter than other brands. AMD CPUs run cooler than Intel parts. Only the R9 290s run hotter than their equivalent nVidia cards and that is only with reference cooling.  He'll be fine.

Don't buy an R9 270X. Get the regular R9 270. You can easily OC a 270 to 270X speeds and they are considerably cheaper. Non reference 270s can be had for $120 on sale. The 270(X) will run most games on high at 1080p no problem. May I suggest looking at an R9 280. They are quite a bit faster and can be had for around the same price as a 270X if you look for sales.

R9 270:

R9 280:

PSU is fine.

Monitor would be a nice upgrade but really I don't notice too much of a difference between 1600X900 and 1080p. I used a 1600X900 monitor for a long time until I jumped to 2560X1440. 1600X900 is a good resolution. I'd save the cash and put it into the GPU.