If you have 2 Voodoo2 cards in SLI, you don't need to bother with a better AGP card. The reason I did mine was because I didn't have a lot of the original components. Like, I had the original motherboard, the original Pentium II, and the original RAM stick. I also had the original case. But that's about it. The original sound card, graphics card, network cards...they're all long gone, and I'm not even sure how I lost them. The AWE64 GOLD card was worth some serious money now, so I'm kind of blah about it. I'd also have used a perfectly working i740 AGP graphics chip just fine with an SLI Voodoo2 setup, as all the heavy-lifting 3D-wise would have been done by the SLI, not the i740.
So, without a decent 2D card for Windows to use and an open AGP slot, I figured what the hell and got the TNT2 on the cheap. It's on-par with the SLI Voodoo2 cards in just about every respect and a little bit faster in some others, especially with 32MB of on-board memory, so it can handle much larger textures. So...when I have a game optimized for GLIDE, I use the Voodoo2s @1024x768 (my default Windows resolution for this system). When DirectX is the only option, I just use the TNT2 card.
In YOUR case, though, I'd just stick with the ATI Rage card you already have and use the Voodoo2 cards full-bore for everything 3D. The ATI's 2D chip should be fantastic in terms of speed and compatibility with old DOS games and Windows's 2D environments. There's not much better except the Matrox Millennium series. Phil probably has better recommendations, but why spend money at this point? You have decent 2D and excellent 3D.
I can't tell what sound card you have, but you have a whole lot of options. If you want DOS compatibility...or to be able to run pure DOS on the system like I do on mine (via exiting Windows 98SE into DOS mode), then an ISA sound card is pretty much a must. There are a FEW PCI sound cards that have a 3-pin connector to the motherboard for ISA-resource assignments to make DOS mode work properly, but the motherboard has to support that, and not very many did. So it's really about the ISA cards.
And, of course, if it were MY retro system, I'd get a ZIP drive for right underneath your floppy drive, because nothing screams out late-'90s like a Zip drive. I was so hype in 1999 to have a built-in ZIP drive instead of that bulky blue Parallel-port Zip drive we had on our older Packard Bell Windows 95 computer, and it really did increase the file transfer rate over the external drive. I used to save everything to those ZIP drives. Now... all of my disks are full with other people's crap. I'm about to dump everything to a CD-R and wipe the drives so I can use them once again.
Also, one suggestion... if you have a home network, get yourself a decent 10/100 PCI Ethernet card that has Windows 98 drivers available. The reason: It's not hard to get Windows 98 talking on your home network. You can then set up Windows File Sharing to have a share folder available. Then, any modern Windows 7+ computer can access that folder and dump files to it. That makes downloading crap off of the internet stupid-simple, as you can download it using a modern browser at speed, then dump the file to your share drive at network-speed (usually around ~12MB/min @100Mbps speeds). If you need help with that kind of configuration in Windows 98, I did mine and got it working properly and I can always be messaged here and I'll usually respond in a day or two to help walk you through it. There's also a lot of other knowledgeable guys on this forum - it's why I come here. They've helped me out a bunch with my DOS configuration, because I refuse to use Phil's "boot prompt" multi-setup DOS thing. I want my system to just boot to Windows or directly to the DOS prompt. I don't want it to sit there waiting on me to choose an option. I want it to just turn on, walk away, make myself a cup of coffee, and come back and it's ready to use.