Can you use another video card to replace the on-board?
Yes.
Usually anyway, but it depends on your motherboard. You need to check the manual or the manufacturer's web site to see if there's a way to disable the onboard video. Sometimes there's a bios setting and sometimes there's a jumper.
Sometimes there's no way to do it, but you can still install another card and then disable (not remove, disable) the onboard card's driver in the Windows Device Manager (it'll have a red X next to it, but that's okay).
Is a Trident 4 Mb card any better than your cheap P.O.S. Sis onboard video?
NO
The Trident is also a cheap P.O.S. video card. Consider something better.
If you're going to replace the it you might as well get a good 2D/3D card and lose the V2 as well.
If money's tight, a V3-2000 usually goes for around $85 retail and $60 2nd hand. A V3 is twice as fast as a single V2 and slightly faster than V2 SLI, plus image quality is a lot better than V2.
You should be able to sell the V2 for $35 used. Lots of people still buying them - mostly people who have an NVidia video card but want a V2 for games that run better on Glide cards like Tribes.
Don't expect too much of an improvement in 2D mode, though. Your CPU is also a bottleneck.
Most video cards perform about equally in 2D mode. The SiS chip is a real junker, though, and usually crippled by crappy drivers. You'll probably see an improvement with a serious video card only because DirectDraw support will be better.
If money's real tight, shop around for a second hand TNT1 video card or an older Matrox card like a Millenium or Mystique G100 or G200. You should be able to get one for a song, and they'll run (and look) a lot better in 2D than that SiS chip (although your V2 will be better for 3D).
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The Flying Penguin 
http://TheFlyingPenguin.com 
Tech Discussion 
is Trident 4mb pci a good vga card? 
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