Hi, Mike2012. Currently, I'm running an old Pentium Dual-Core with an HD 7750, and I'm unable to run it at 60fps online at the lowest settings. However, on campaign I can get 20-50fps on ultra at 1280x720.
Now, I was thinking of getting a new XPS for around $800 with a core i5 processor, but is it worth it to just play BF3?
Or would it just be better to get a new PSU and a GTX 560 Ti for around $500? Or will I still be bottlenecked by the Dual Core even with a high end graphics card?
My Intel Q6600 bottlenecked my single 560Ti...so I would say yes, your processor will bottleneck you card (indirectly).
If we're only talking BF3, then your procecessor is at times bottlenecking your entire system. BF3 is quite CPU intensive and regardless of you GPU, will output lower frames depending on the speed and the number of cores on your processor.
To go further, BF3 Mulitplayer will really test a system (say you go in the biggest map with 64 players). CPU handles all the logistics (from player positions to physics, etc.), GPU handles the textures and effects, and HDD/SSD will shoot textures to your RAM/VRAM. All this to say that BF3 will maximize most components on your PC.
CPU: when entering a full server (64 people), there's a lot of movement/physics occuring at once. Furthermore, there's generally a lot more going on at once and therefore will stress your CPU more than a typical single player level.
GPU: If set to higher settings, the game needs to load->transfer->output textures, effects (code OLE from DX and the Frostbyte engine). All these will stress your GPU and playing with the settings should help you get the perfect visual versus FPS (Performance). AMD or Nvidia usually have recommendations based on the GPU model you have.
HDD/SDD: On bigger maps, although most textures a loaded on to your RAM/VRAM, it's impossible to load everything. So, occasionally, your PC will read from the source (your hard drive). If you have a slow HDD then you might want to consider an upgrade. Best way to figure out whether you're HD is slow or not is by the time which it takes to load a level before playing. If it's ridiculiously slow, then you have your answer. I've gotten a SSD drive for improving loading time and it immidiatly reduced the amount of stuttering I was previously experiencing in games like BF3, Skyrim, and other open world/huge map games. Highly recommended upgrade!
That said, I don't have an answer for you. If I had to choose one upgrade, it'd be the CPU (but that means a new motherboard, possibly ram, etc.).
How fast is your CPU? You can get a Dual or even a Quad core CPU that could be a lot faster for your current system for pretty cheap, 30-40-50 instead of the 800 for a new computer. You also did not say how much ram you have.
3 gig CPU, 4+ gig of RAM and try things with your 7750, should run pretty well, although not at great details. Shadows kill a system, so check your settings, you can usually get higher quality textures and AA and more FPS if you turn off shadows on the same system with no upgrades. If you see no change with the new CPU, then you can look for a faster card.