Rune will indeed run awfully on that CPU no matter what card you install. It's based on Unreal Engine and that was a very CPU demanding engine at the time. Keep in mind that despite the megahertz disparity, your CPU is not far off of a Pentium II 266 or Pentium II 300.
Your best bet is probably a Riva TNT or TNT2 card. The Riva TNT is about on par with a TNT2 M64 and the TNT2 is about twice as fast, but you won't really be able to tell a difference with that CPU for the most part, especially if you only intend to play at low resolutions such as 640x480 and 800x600. IMO the RIVA 128 is a little slow even for this CPU, TNT is twice as fast and probably about as easy/hard to find. Not a bad choice, but still...
A GeForce 2 MX200/MX400 or GeForce4 MX420/440 would also be a nice choice because they offer T&L which could help massively in some games (MDK2 would be pretty playable for example).
On the ATi camp, be on the lookout for Radeon 7000/7200/7500 and perhaps even 9000/9200 PCI cards.
In general, just be patient and be on the lookout on eBay and Amibay, I'm sure something will come your way. Electromyne.de also has a few cards, but they are very expensive, you could use Phil's Computer Lab discount code to make it more bearable.