Welcome.
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ISA cards are not my thing but if you look around there are several threads that discuss them.
Here is one: ISA graphic card, good stuff or not ?
I'm rather surprised there aren't several people in here already. Usually a popular topic.
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CL 5424 is not a slow chip for its days, especially since it must be on VL bus. If I remember correctly 5424 was the VL version of 5422. This is not a subject that I'm well versed, but I think by "changing" your existing onboard VL bus graphics with an ISA one, there is a very good chance of your performance getting worse.

To tell you the truth, Duke Nukem 3D was a 1996 and Blood was an 1997 game. I was playing Duke Nukem on a 486 during the times, but it was a Cyrix 5x86 120 with a PCI 2MB S3 Trio graphics adapter, and it was still not in high resolution. I never played Blood, but by 1997, I already have my first Pentium. Normal Pentium 200 was already in market by 1996, and MMX 200 by 1997, and PCI S3 graphics cards was all the rage. So, the performance for this games you are getting from your machine is probably what you should expect.

PS: I just checked and see that Duke requires a 486 DX2 66 and Blood requires a Pentium 75 as a MINIMUM, so, really don't expect much.

If the onboard video is on VLB bus and the motherboard does not have a VLB slot then sorry, it is maxed out as far as video performance goes. Those games you wanna play would be better off with a Pentium anyway. If you REALLY wants to improve performance then you van try a Pentium Overdrive.

Played Duke3d on a 486/66 with a 2MB VLB ATI Mach64 onboard back in the day. Even then the PC struggled on lowest settings if a lot was happening on the screen or large maps.
ROTT may be last of the big name 3d shooters that didn't need a Pentium?

Thanks for the information. My second DOS machine has a Pentium II 300Mhz in it, so I should just use that for such things. I guess that is probably too fast really for a DOS machine. Maybe some kind of Pentium 1 CPU would be best? Not sure what motherboard that system has right now. I wonder if it would be better to put a slower Pentium in there if possible or just leave it? I might try and find out what the slowest CPU is.

P2 or (faster) would make a great dos PC for anything from the mid 90's By then most games are no longer affected by system speed.
As long as you still have a ISA slot for sound and as a bonus plenty of AGP cards have bester VESA compatibility then alot of ISA cards.
It'll definitely make a great Duke3d PC if you want to play in SVGA

Great! It sounds like I have all of my bases covered for now.