The 3GB of memory is probably holding you up the most when it comes to add ons.
Overclocking a 1st gen is a little strange. Memory and BCLK are tied at the hip so you have to overclock/underclock the memory simultaneously. I had little luck getting the early Corsair Dominator 1600 (which is likely what you have) to overclock much. (stock was 1333, so it is already overclocked)
So BCLK is 133Mhz * 12 = 1600
What you want to do is change the memory multiplier to a lower value, then increase the BCLK
So set it to 10, then try a BCLK of 160, this brings the memory up to 1600 again, but the CPU (BCLKx20) up to 3.2Ghz, at the same time the higher frequencies.
You can go to extremes and lower the memory multiplier further, increase the BCLK to even higher levels (200 was fairly common) and use lower CPU multipliers. To get 3.6Ghz etc.
Here is a detailed guide:
http://www.overclock.net/t/538439/guide-to-overclocking-the-core-i7-920-or-930-to-4-0ghz
As for a graphics card, you currently have a fairly entry level card. GTS250 is at the top of the consumer range 6/7 generations back. You could effectively triple graphics performance with something like a GTX750ti or GTX950. But you machine can handle any of the high end cards available today. Not really sure how you ended up with a 750W supply in that config, but it will certainly come in handy.
GTX970 is the goto Nvidia card for 1920x1080 gaming of the latest titles. That or an R9-390 from AMD for 1920x1080+
Beyond that is the GTX980ti and AMD Fury X, but then your CPU will start to show its age. Really comes down to your budget.
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