Your resolution is on the borderline where your GPU is going to be hitting it's FPS ceiling before the CPU does.
*Please disable Vsync for these tests -- Also, try to repeat each test in the same environment / situation*
What kind of FPS do you get in your current games at native res (1680x1050)?
Try moving the resolution down to 1024x768. Has your average FPS increased at all?
If yes, you've still got room to improve on your GPU before your CPU becomes on the bottleneck.
If no, then your CPU is holding your graphics card back.
Even if your CPU is bottlenecking your current setup, or will be the bottleneck, in your new system. You will see a definitive increase in performance from just a GPU upgrade. If you're not happy with the increase, you can probably pick up a 5000+ in the $100 range and that would be compatible with your AM2 socket MB.
I'd like to upgrade my graphics card, but I'm worried that the processor will be a bottleneck and possibly the PSU if I decide to go highend (8800GTX/GT or 3850/3870).
A 550W PSU will be more than enough to power any single card. (You really want to concern your self with the Amperes on the +12V rail when checking power requirements -- but you have nothing to worry about with that Antec PSU)
As for the GPU - I'd go for an 8800GT just because the price is unbeatable right now. It's probably better performance / $ than the 3870 atm. (Also, try to find one with a dual-slot or after market cooler already attached. ) I would not recommend that you go with the GTX at the moment, it's old and offers minimal gains vs. the GTS / GT.
I edited my post above to say this (Just to be more clear ):
Even if your CPU is bottlenecking your current setup, or will be the bottleneck, in your new system. You will see a definitive increase in performance from just a GPU upgrade. However, if you're not happy with the increase, you can probably pick up an X2 5000+ in the $100 range and that would be compatible with your AM2 socket MB.
I completly agree with rgeist554's post just upgrade the Graphics card see what kind of results you get. You now know how to test to see where you are as far as the CPU performance is. If your not happy upgrade that also. Just to be sure you are looking for a G92 version of the GT/GTS these come with 512 MB ram. The older model GTS (G80 core) come with 320/640 mb ram. Im begining to look like a MSI sales rep as i have been recomending this card all over the forum of late but i havent seen anything better Performance/money yet.(In my own opinion) http://www.newegg.com/Product/Product.aspx?Item=N82E168...
Good call that man (rgeist554)
$100 no, but $50 yes. In those two links of yours, both have decent cooling but the GTS will be faster stock and overclocked. Plus it comes with the Witcher game and that GT doesn't right now. ANyway, all things considered it's worth $50 for sure to go with card A over card B. If the game means nothing to you, either is a good choice.