Posted 21 October 2011 - 08:25 PM
However, to answer the question directly:
The 7900GS is an older-generation video card by nVidia
It was, at the time, relatively high-end, had a 256-bit memory bus (one of the highest memory bus-widths available), only 256 MB of memory (which today is a little low), and supported DirectX 9. Some cards had a 512 MB RAM quantity, but those were rare.
The 5450, in comparison, is a next-to-newest generation budget video card by ATI.
It has a 64-bit memory bus (the smallest available in modern cards today), 512 MB or 1 GB of RAM (mostly a formality, with a 64-bit memory bus the card cannot access the memory fast enough most likely), and supports DirectX 11.
It is generally sold as a HTPC (home theatre PC) card, or as a budget gaming card. It will do some newer games, but not really much. Optimized more for Blu-Ray playback.
My recommendation would be the 5450 if you're looking for Blu-Ray or low-heat (the 5450 will probably put out less heat than the 7900GS).
I would recommend the 7900GS if you're looking for higher-end gaming (even with its lower specs it will outperform the 5450 in every way.)
Make sure both are PCI-Express, while the 7900 should be, it's possible you could have an AGP model. I don't recall if AGP versions of teh 7900 were ever sold.
Make sure also your computer is PCI-Express.