Iirc the limit was 512MB ram. www.msfn.org forum has a LOT of info about tweaking 98 and it's limits.
Thyere 'should' be ways to get 98SE and ME to work with more then a gig of memory in total.

some of the sites that sell it show it works with win98.

Now the thing you may read over as MSFN is that windows98se was supposed to work fine with 2GB total system memory, later MS noted that it really only works with 1GB Ram before bigger issues happen. At that most things I have found is that W98SE works best with 512MB or less of system memory. There are patches but can still run into issues with more than 512MB. I personally use 768MB without issue and a 128MB Geforce 4 Ti 4600, and 2 Voodoo 2's in SLI 12MB.

To be safe I'd use only 512mb system memory for your vp990 for best reliability of W98SE. Make sure to set MaxFileCache to the appropriate setting. should be 70% of system memory unless running 768mb or more.

By the way, the thread starter asked about big VRAM, not main RAM.

As for the VRAM issue, I don't think Windows 98 is affected -- I have ever installed Windows 98 with 128MB card and it was flawless.