So what can i do?

 

I would suggest something like:

 

1.- Try those addons in your current system

2.- If you don't like the results, get a GTX750 Ti. It's a very low TDP card and may work with your power supply. It's a considerable upgrade from an HD6450

3.- With all those addons you will still need to turn down your settings unless you also upgrade your CPU & motherboard. If you're still not happy with how it looks or performs you can either:

 

a) Check if you can just swap CPU (I5-3570K) & motherboard & cannibalize the rest of the components. You'll need to make sure your Optiplex is not using some proprietary form factor for it's motherboard and/or case. A quick google search ... https://www.dell.com/downloads/global/products/optix/en/opti_755_techspecs.pdf there seem to be 4 different Optiplex 755. The SFF is most definitely not upgradeable 

 

b ) probably better, just get a new case and a good PSU and build a new system with an I5 4670K and a GTX 750Ti or GTX 760  

Peak and tweak or start throwing money at new hardware.

how about on this config?

 

i5-4440 3.1 GHz

NVIDIA GeForce GTX 660 2 GB

1 TB HD

Windows XP (I will never upgrade from XP! XP is the best OS ever!)

8 GB RAM

Looks good to me, you shouldn't have many issues. Be careful on XP, since it no longer is supported by the manufacturer, many companies are also ditching support for it. This includes PMDG, who doesn't support XP on their latest project, the PMDG 777. My processor, other than being an i7, is very similar to yours, the clock speeds only differ slightly. The one listed in my PC is the one listed at max potential when intel turboboost activates.

Also if your WinXP is 32bit a 2GB video card won't work. You'll eat Virtual Address Space (VAS) faster then you can say Swiss cheese!!

 

Edit: I'm a little confused, your opening post said you used Win7 Pro 64 Bit??

Windows XP?

Scrap it

Windows 7 64 bit is a must

I use XP 64-bit edition.