I have built an old DOS computer to play classic games on and found out that there are many differences between video cards (Yes, even between 2D cards). Some of them tend to have problems when it comes to scrolling (best noticable with horizontal scrolling).
The Matrox cards (including Mystique, G400 etc.) seem to have big problems with Commander Keen (episodes IV through VI). I never got that game working correct with a Matrox card. Same goes for cards based on Tseng ET6000 chipset (I tested with Hercules Dynamite 128), although that could be fixed with the game's option "Fix jerky motion", I believe.
With Jazz Jackrabbit, horizontal scrolling is not very smooth (jaggy) when using a Matrox Mystique for example. I am almost sure, that the G400 series will show same results. With an ET6000-based Dynamite 128, Jazz Jackrabbit is not playable at all. (total screen mess when starting a level).
What really works well for 2D games seems to be the chipsets from S3 graphics (ViRGE chipset or TrioV64). Until now, I didn't have any graphics issues with 2D games using this card.
Another important thing when it comes to DOS games is VESA 2.0 compatibility and good support tools (e.g. refresh rate setter utilities for VESA modes under DOS). You won't like 640x480 at 60 Hz, it's just not healthy. ;-)
I am quite happy with my S3 ViRGE DX under DOS, although I don't like the Windows 3.11 driver. It seems to have problems with changing between mouse cursors (pointer disappears for a sec when changing between cursors (text cursor, window resize etc.), but there maybe a solution to that problem I don't know of, yet.
ViRGE- or Trio-based cards should be fairly easy to purchase on eBay. Here in germany, they are quite common. Maybe that's because of their good compatibility.
If that is not enough for you, you can always combine it with a 3Dfx add-on card (e.g. Voodoo, Voodoo2).