With only a 250W power supply you may be pushing the limit with two video cards, but it's hard to be sure. Neither of the video cards you're planning on using are particularly power hungry. I suspect nether comes with a fan, and the GeForce2 MX may not even have a heat sink. The PCI connector can only supply 25W and I believe the AGP 1.0 interface had the same limit, so this sets an upper limit on what these cards can use, but it's likely both cards used significantly less power.
Since I assume you're using an ATX power supply, if necessary you can just replace it with a newer bigger one. You'll have to look closely at the specs though, since modern supplies use the +12V rails to supply most of their power, and may actually supply less power through the +3.3V and +5V rails then what you're using now. A modern supply with also lack -5V, but only really ancient hardware used it, nothing in your PC will need it.
Drivers shouldn't be a problem, since the video cards are made by two different manufacturers. Using two video cards from the same manufacturer often didn't work on Windows 98, but you should be fine with a 3dfx and a Nvidia card installed at the same time.
The VGA port conflict TEMLIB mentioned should't be a problem, as only the video card you designate as the primary video card in the BIOS setup will provide VGA backwards compatibility. The secondary video card will have its VGA compatibility disabled. The primary card will be the one that's used while booting, and it should be the one Windows 98 uses as the primary monitor.
Since you're only using one monitor and since most games didn't give a choice of what display adapter to use, you'll probably have to select which video card you want to use by rebooting your PC and changing which video card is primary in the BIOS. I'm not sure how Windows 98 multi-monitor support will deal with this. Ideally it'll treat your secondary display as disabled, since it doesn't have a monitor connected to it. If not you'll want to manually disable the second display, otherwise your mouse pointer will end up getting lost on it.
You might also run into Windows 98 needing to reconfigure things a startup after switching which video card is primary and asking you to reboot multiple times. In that case I'd would just keep saying "no" until all the pop ups are done and then rebooting just once.
I just did a quick check of using Windows 98 two video cards, an AGP Radeon 9550 and PCI Rage II+ and it worked fine as far as I tested. I was able to use both simultaneously, though I had to manually enable the Rage II+ as a second monitor. Despite being from the same manufacturer there were no driver conflicts, as they're from two very different generations. I didn't test it with any games, as I only have DOS games installed on that PC.