On the topic of the SH4+FPU compared to the Pentium III. My Test Results with the PIII 800 and 3DMARK 2000 and 2001 show the peak polygon throughput with one light at less than 3million polys per second. If anything I'd say this puts the PIII 800 plus Direct X / Windows overhead fairly close to a "real world" Dreamcast peak as shown in only a few games. This is still far short of the 3.5mpps spec given by Sega or the 5mpps spec given by Melbourne House for TD Le Mans. At 533mhz the PIII "only" peaks at around 2-2.2mpps in 3DMARK.
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I tried to install the Neon 250 in my Athlon XP or Pentium 4 systems to see if the "Pixel Perfect" bios and drivers from 2000 do anything different, but both have a newer AGP port with an extra divider in them, making my Neon 250 physically incompatible with systems newer than the ASUS P3B-F.
On the topic of GTA III. The original discs I have are two CD-ROMS, nothing over 600MB and the actual install folder isn't much more than a CD as well. Which means that *something* must stream off of the optical drive on the original PC version.
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Finally, GTA III has never impressed me in anything about its scope, single load city size, objects on screen or especially texture detail. After playing Shenmue 1 + 2, Crazy Taxy *2* and Super Runabout I never saw any reason why GTA III could not be made to run, with better image quality on the Dreamcast. Given the poor resources given by 3rd parties to most Dreamcast titles, I'd expect it to have performance issues and more pop in sure, but it'd be playable with better up front image quality. I've seen plenty of analyses showing that with VQ textures the Dreamcast effectively has more than 32MB of RAM available besides.
GTA II
can be made to run on an 8MB mobile processor, it really is_not that impressive of an engine. But I am 1000% certain the legend of it's minimum system requirements mystically being the PS2 will persist.