I'd really say the Dreamcast's performance depended too much on the type of game. Have a game with minimal overdraw, such as a flight sim, and you have worse performance than a voodoo 2.
I dunno, the DC should significantly outperform the Voodoo 2 even at this.
My Voodoo 2 was 93mhz, with 93 mp fill rate, 16 bit colour with 12MB Vram.
The DC was 100 mhz with 100 mp fill rate, 32-bit internal colour that looked
enormously better than a Voodoo 2 even when dithered down to 16-bit, and it could draw more than twice the number of polygons per second. Taking into account texture compression, it should also have a higher variety of better looking textures despite having less video memory.
So that's fill rate, polygons and textures in the DCs favour, as well as a 32-bit post-transform store for geometry (while the Voodoo 2s used a 16-bit Z-buffer). Its a wash in favour of the DC's GPU - the only possible exceptions I can think of might be resolution (my Voodoo 2 struggled with 1024 x 768, but it did it) and multi-textured fill rate.
Dreamcast was at least in range of the top end 1998/1999 cards however, I'd say its cpu and amount of memory were greater limitations in the types of games it could do than anything else.
The CPU was actually an enormous strength if you look back to when the DC was made, it's what allowed it to calculate so many polygons for its time. The DC was beating in-game what my Pentium 2 400 couldn't even match in 3Dmark's "high poly" test!
I don't care how good the FPU was, I can't see anything SH-4 based running at 200mhz holding a candle to a 500mhz athlon, and I think memory was divided into 8MB system (pretty low)...
That was actually 16MB (plus another 2MB audio), so not low at all for the time - in fact it was pretty impressive by console standards in 1998. To put it in perspective the DC had 6.5X the memory of the N64 which was released two years earlier, while the Xbox three years later had about 2.3X as much. The GC only had more memory if you include its tremendously slow A-Ram.
I suppose this shows that at 40MB for an early 2000 console the PS2 was a bit of a RAM beast too. Of course, main memory is one area where PCs always have dominated consoles.