Looking for tips to upgrade my pc spec

I Just want to upgrade for more smoother instanced to mulitbox. I have no problem running it with 2-3 character on Diablo 3 but 4th will make it extreme lag. I probably would like to spend around 300-500$ to upgrade. I plan to multibox ff14, also I did try 2 instance and it runs fine no problem during beta. This pc that I built is about 3 years old.

Processor: Amd Phenom II x4 965 3.4 ghz
Motherboard: ASUS M479XTD EVO AM3 socket for processor
Memory: 8gb
Harddrive: Western Digital 500 gb Sata
Videocard: Nvidia Geforce GTS 450 1 gb
Operating System: window 7 home edition 64 bit
Power Supply: ThermalLake 600 watt
Monitor: 1440x900 resolution 19 inch LCD ASUS

I'm thinking about upgrading my monitor so I can see more to multibox however I will probably need stronger spec for that as well. 1980x1080 or 1980x1200 resolution? Which one would look better along with 23ish inch monitor. Any advice would be greatly appreciated.

EDIT:
I forgot to mention that I think I should upgrade video card because I checked performance chart and it says low for this current graphic card that I have right now. I can see that I can buy 3 times more performance GPU for 250$(Geforce GTX 760) according to this benchmark chart. (GTS 450 with 1533 Benchmark vs geforce gtx760 4986 Benchmark) http://www.videocardbenchmark.net/high_end_gpus.html

[MiRai]

It all depends on where your bottleneck is and what the rest of your components look like when you're seeing issues with FPS. If your GPU is maxing out at 100% while your CPU is sitting at 90%, then an expensive video card is only going to be good for 10% of CPU before you're hitting a bottleneck again, so you should be looking at this page in the Wiki to see what your hardware is doing.

And keep in mind that if you upgrade your monitor you're going be pushing more pixels (and adding more load) unless you keep your game clients at the same resolution that you're using now.

EDIT: I forgot to mention that I think I should upgrade video card because I checked performance chart and it says low for this current graphic card that I have right now. I can see that I can buy 3 times more performance GPU for 250$(Geforce GTX 760) according to this benchmark chart. (GTS 450 with 1533 Benchmark vs geforce gtx760 4986 Benchmark) http://www.videocardbenchmark.net/high_end_gpus.html

While I'll agree that the GTS 450 is a low-end video card at this point in time, I wouldn't use that site to pick a video card for a game because those "points" that they give the video cards do not translate to FPS in games.

Ok, Thanks for the bottleneck program to monitor. I load up 3 instance for diablo 3. While i'm just sitting in main menu, all of my 4 cpu core load stays under 10% average. My memory load is 36% with 5.1 gb available. As for gpu core its at 85% while im doing nothing. Also, when I switch to different instance, the GPU core goes to 99% instant, my CPU core goes high 20's, my memory stays the same. So it seems that my GPU is slowing me down? Would it help if I save a report so you can look at it?

EDIT: Also I put lowest setting for each instance.

I saw a video that u multibox 5 character on ff14, I was wondering which videocard you were using?

[MiRai]

Also, when I switch to different instance, the GPU core goes to 99% instant, my CPU core goes high 20's, my memory stays the same. So it seems that my GPU is slowing me down? Would it help if I save a report so you can look at it?

EDIT: Also I put lowest setting for each instance.

As I said before, the GTS 450 was not a very powerful card for multiboxing when it was current (not trying to make you feel bad or anything, it's just a statement) so it was likely to be the problem; and the hardware monitoring program is showing you that.

I saw a video that u multibox 5 character on ff14, I was wondering which videocard you were using?

viewtopic.php?p=19230#p19230

Real question is... what's your budget? Once we know what that is, then ppl know what kind of card, etc to recommend.