3d mrk is whats known as a synthetic benchmark, so take your results with a grain of salt, another thing it does when it compiles the information given to you is it present it in a format to show you what people wiht the same setup have achieved.
In your case it looks like your 750 is at stock clocks (not overclocked) whilst the vast majority of people who would hve been benchmarking vantage with a 750 would have had it overclocked, the 750 was a very decent overclocker in its hey day.
It looks like your 4870 is still going strong, it and the performance of the cpu combined is what made the score so vastly different. Laptop cpu's tend to be more energy efficient and this means they use a lower voltage so tend to not clock as high for general or turbo use. whilst the processor is a lot more efficient at lower clocks it looks like the 6770 is letting it down, the 4870 is a strong compute gpu whilst the 6770 is basically a gpu refresh of the 5770 which was an ok performer but i would put it under the 4870 in terms of compute ability...
With your current setups, your desktop is would be more balanced for gaming but the performance might not be up to par, a new gpu and a slight overclock would help lift its game a little.
Because of the bottleneck at the gpu end in your laptop, it should be ok for most gaming, but maybe not on external monitors/3d/aa+/high settings etc, the cpu will be able to make up for the short comings of the gpu, but you will only get so far.
for your price point, your a bit in luck, as due to the keplar domination, AMD/ATI prices have dropped, so price wise you have high-end 560 Ti's from nvidia in the same price bracket as a 7850, which is between the 200-300 price points. The 7850 isnt too bad but gets the edge on the 560, plus if you can stretch it a 7870 should be ok to. Both these cards should perform fine on a single 1080p screen, regardless of settings. If your looking at going multiscreens i would highly recommend a cpu cooler and overclocking the 750 for a few more mhz inorder to release those frames!
Nolite te bastardes carborundorum