On paper, based on the fill rate only, all the high-end graphics adapters available on PC in 1999 were at least as powerful as the Dreamcast’s PowerVR2:
- the CLX2 in the Dreamcast ran at 100MHz, with one operation/pixel/texture per clock, and memory bandwidth of 0.8GB/s;
- the PMX1 (PC-compatible PowerVR2) ran at 125MHz, again with one operation per clock, and memory bandwidth of 1GB/s;
- the Riva TNT ran at 90MHz with two operations per clock and memory bandwidth of 1.76GB/s;
- the Riva TNT2 (March 1999) started at 125MHz with two operations per clock and memory bandwidth of 2.4GB/s and up;
- the Rage 128 started at 103MHz with two operations per clock and memory bandwidths of 1.648GB/s and up;
- the “special” Rage Fury MAXX ran at 125MHz with two graphics processors (on Windows 98 only), for four pixels per clock and 4.576GB/s in ideal circumstances;
- the Voodoo 3 started at 125MHz with one operation per clock but two texture units (in DirectX games), with memory bandwidths of 2.288GB/s and up;
- the GeForce 256 SDR (October 1999) ran at 120MHz with four operations per clock, memory bandwidth of 2.656GB/s, and hardware T&L.
However the PowerVR2 and SH-4 combination could provide excellent results in games written appropriately, using the PowerVR2 triangle and tile-based rendering pipeline and the SH-4’s arithmetic operations (dot product, inverse square root). This results in quoted speeds of 7M (opaque) polys per second.
To reach that using a PC GPU only, you’d need to wait for the GeForce 256 in late 1999, with 10M polys per second. However a mid-range Pentium II with any of the above GPUs could perform the same calculations in software, and 1999 was the year of the Pentium III and Athlon, providing ever-higher levels of CPU performance.
Perhaps the safest way to answer the question is to look at the specs for PC ports of Dreamcast games. Crazy Taxi was released on PC in 2002 and requires a 500MHz Pentium III or equivalent with a DirectX 8-compatible GPU — of the PC graphics adapters available in 1999, only the GeForce 256 is officially supported. That suggests that a 500MHz Pentium III or Athlon with a GeForce 256 could at least equal the Dreamcast even on a game designed for the Dreamcast.
My memories of games in the day suggest that a lesser PC could produce better graphics than the Dreamcast, at similar resolutions, assuming PC-specific optimisations: a high-end Pentium II (450MHz), or mid-range Pentium III (300MHz and up), or Athlon (the slowest ran at 500MHz...), paired with a TNT2 would have been sufficient to outperform the Dreamcast.