Pre-Athlon, there were some "authorities" (Millward, for one) who said AMD rigs were not good for Cubase. There were also some early VIA chipset problems on some AMD boards.
As of today, I don't know of any real Cubase troubles with AMD CPUs or better motherboards.
There are certainly a few Cubase bugs, but these do not seem to have anything to do with AMD CPUs.
I am using a 1.4GHz/266 Athlon in an ASUS A7M266 motherboard (with the AMD 761 chipset) with 512M of Crucial DDR memory.
While I am still struggling with learning Cubase, as far as I know I have never had any trouble resulting this CPU/MB/Memory combo. I researched pretty heavily before buying this setup and on the hole this was considered a good platform for what I was trying to do. There are cheaper motherboards, but this one has a lot of people using it for recording with good results.
Unfortunately, memory has taken a HUGE jump in price in the past couple of weeks, but everything else seems affordable.
This is the best I can do for you, but you might want to check on the forums at
www.cubase.net for more expert opinion.
Hope this helps.