If you want to upgrade the actual look of the game, there is an AMAZING texture pack out by QARL. But, just make sure your graphics card can handle it!

There are also some good no-tiling texture packs and some other this and that available. One of my favorite was a lighting mod that lit up windows and torches in the cities but sucked up a lot of CPU usage.

Of course, the game is now, what, 6 or 7 years old? So any machine built in the last year or two can probably run all that at max max.