As wikipedia says, A2 requires host OS support for write caching and command queueing, so its likely A2 isnt the best idea. A1 doesnt need that.
Personally i'd look for an A1 card, that also has a decent sequential write rating, such as U3. This combination should hopefully get a card as fast as possible. However there will still be variations between cards. Theres a few sites around that are benchmarking card performance on RPi platform, so finding a card that performs well there will give best performance on a PC as well.
And yes, the old IDE interface and/or SD to IDE converter chipsets will limit the max throughput. Realistically though, its the random IO performance that you "feel" when using a PC, especially something retro, where the spinning disks of old were really slow anyway, and files are small so your never needing lots of sequential IO.
Sequential reads/writes arent even an indicator of random performance, and infact i've seen cards personally where they had slower sequential read/write, but much faster random IO.