Yes, that SIS is your video card. It sounds like one of the built-in video chipsets.
To check, right-click on My Computer and choose Properties. Now look in the device manager tab. Under Display Adapters it will tell you the name of your video card.
There are two problems limiting your video speed. The first is your video chipset, as it is not one with good 3D rendering power. The second is your CPU. The AMD K6 series does not perform well in certain types of games, notably flight sims. The AMD K6 does particularly poorly in Aces High, as AH uses the Floating Point Unit in the CPU heavily, and this is the weak point of the K6 CPU.
The easiest upgrade is likely your video card. If you are not experienced, I would take your machine to a professional and ask what your options are for an upgraded video card. Find out if you can use an AGP video card, of if you need a PCI one. Then come back and let me know, and I can make some reccomendations.
BTW, I am a techie geek by profession.
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Lephturn - Chief Trainer
A member of The Flying Pigs
http://www.flyingpigs.com 
"My P-47 is a pretty good ship, she took a round coming 'cross the Channel last trip.
Just thinking 'bout my baby and lettin' her rip, always got me through so far."
- Steve Earl
