This is my current system Information any advise would be appreciated I know I will also have to replace my PSU will my CPU be able to cope with upgrade,
Windows Vista Service Pack 2.
Intel Core (TM)2 Quad CPU Q6600 2.4GHZ with 3GB of ram and I am currently using a Nvidia Geforce 8800 GT
There isn't really much to upgrade on your PC. Your motherboard is probably limited to a 1066mhz fsb CPU, so there is really no better CPU for you to spend your money on, at least being worth the extra money for a minor increase in performance. You can bump up the memory to 4-8GB, upgrade to windows 7, and upgrade to a video card like the R7-260X. That should give you a PC able to play games at a reasonable setting. If you didn't have a dell motherboard, you could overclock that CPU to 3.4Ghz, I did it on my old system with a Biostar board.
I have the same system and just updated to a GTX750Ti. No problem with upgrade. The GTX750Ti uses fewer watts than the GTS540 that I had before (after a previous upgrade from the GT8800). Performance in games is about 50% better than with the GTS540 (in terms of FPS). I can run some new games at high or ultra settings with decent FPS, which is really nice.
However, the 3GB limitation of Windows Vista 32bit is a big pain as often the memory is the bottleneck. I am considering upgrading to Windows 8.1 64bit with some extra memory (to get 6GB).