In this topic, a lot of misinformations have written about the SiS 6326, especially about the performance of these cards.
Indeed every card can vary in performance based on the RAM and the clock speeds - there was a lot of manufacturers selling suboptimal video cards from of every chip vendors, including ati, s3, and nvidia products, and of course from sis products as well. So we can only talk about a typical card (faster and slower specimen will also exist).
About the performance of the SiS6326 (8 mb, agp, later revisions). The SiS6326 - being a chipset from 1997 - was a mid-range card, with midrange performance.
If we consider the rush, the riva128, the permedia2 as high-end, the s3 virge and ati rage2+ low end, then we can say the sis6326 is a midrange card, with performance in between this two (but closer to the high-end than to the low-end).
The Cirrus Logic Laguna3D was also released in 1997, and the SiS 6326 performs about the same, but usually faster than it by a good 20-25%.
The SiS6326 is typically about 30% slower than a Voodoo Rush, and about 40% slower than a Riva 128. But in some titles, the performance is equal, or even faster than the Rush.
The performance of the SiS 6326 almost perfect in 1997. However, the architecture doesn't handles well the games released in late 1999 and afterwards, in these games, the speed deficit is more significant compared to the riva128. The reason for this is unknown to me, its either the higher polygon count, the fill rate, or just quality of drivers which gets hogged due to the increased number of the draw calls, or some other reason.
The card works well with Cyrix, P1, Winchip, K6 processors (ALi chipset will however cause stability issues if the AGP driver is not installed). The driver overhead is relatively low. The card will however NOT scale above this point! So it makes no sense to use hem in systems with 333 MHz+ Celerons, Pentium2 or Pentium 3 processors.
In contrast, a TNT1 will scale very well when going from a 200 mhz cpu to 500 mhz, but a SiS6326 - similarly to cards like the Laguna3D, or even the Voodoo Rush - will not produce any notable scaling, as it already hits the wall.
The SiS 6326 supports 24 bit rendering, but to keep the performance high, its recommended to use 16 bit rendering instead.
The card doesn't have enough performance to deliver acceptible 3D performance above 640x480, i actually used it mostly in 640x400 when i had it as a main card.
The card has low picture quality in 800x600 or above, due to the weak ramdac, in 1024x768 the picture is very blurry.
The card has no reliable opengl support. The beta opengl driver only usable with opengl 1.0 based titles (quake2 and such), but opengl 1.1 (quake3) is not compatible, or at least it was still not compiatlbe, when i have replaced this card, i am not aware of the drivers after i have replaced the card. It might got driver updates, in fact, it probably got, as the card was also used on motherboards, so SiS had to support it, even into the XP era.
The built in XP drivers are acceptible, and usually they work better than the win9x drivers (or at least that was my impression).
Overclocking is not possible, the card doesn't have notable overclocking potential.
In short, its better choice than a virge3dimage/rage2+, but a little bit worse than a riva128/permedia2/voodoo rush.
Video: SiS 6326 compared to other cards in the era with a Cyrix: https://www.youtube.com/watch?v=IcT-c4MpIvs
