If you are talking Q2 (and I think Rainbow6 uses the Q2 engine), then the Voodoo3 will be the best. It supoports 3DNow in Quake2 for a pretty good jump in performance. If you can wait a bit the Voodoo4 might work too. Anand disabled half a Voodoo5 with beta drivers and showed the Voodoo4 at 166Mhz beat the Voodoo3 350 without any trouble and ran really good in games like UT. It should smoke in Quake2 and 3 for a resonable price.
Right now the fastst for about your price is a GeForce256 DDR ($202US+S&H) and can overclock descent (about 150/350), putting its performance very close to the GeForce2 GTS (at atock speeds) in 32bit color or resolutions at 800x600 or less (the GTS is hamstrung by the 333Mhz DDR).
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