K7S5A with DDR266 is close to KT266a performance. With PC133, it loses any performance advantages. Possibly a slightly better memory controller than the KT133A, but not significant. You won't lose any performance by getting the board now and using PC133 with the much faster problem, and then you can upgrade to DDR later, but your only gain for now would be the faster processor.
The KT133 doesn't support a 133MHz bus technically, so if the A7V133 uses that and not the A version, then he wouldn't be able to use an XP2000+.
I use an early K7S5A, and it hasn't been a "bad" board, but it hasn't been perfect. I seem to have problems with floppies being corrupted, despite the issue of corrupted floppies under Win2k/XP having been corrected with a new BIOS. There were also issues with voltages, but I think that's fixed in new versions of the board. ECS just generally seemed to have issues with consistency. Lot of bad boards, lot of perfectly good boards.
You might want to verify exactly which board you're getting. ECS has different versions listed on their site, all K7S5A, but not the same manuals or BIOS. I'm not sure what the differences are.
The forums here have a dedicated ECS forum (they have archived the pre-2002 posts to a separate forum, which may have the majority of the K7S5A threads, so you may want to look at both). You can read up on this board if you want to be sure it's the one you want.