From my past experience, your concern is quite justified!
I used to have an ATI Rage IIc. Under Win98se this had a great driver
with all kinds of oprions and modes. I usually ran 32 bit colour and
1024 x 768.
When we upgraded to WinXP, the only resolution available as 800 x
600??? WTF!
Went to the ATI website for an updated driver......
"Driver is included with WinXP click here...
www.mircosoft.com ...."
Gee, thanks for NOTHING!
SO I upgraded our video cards.... to nVIDIA!
I would not be surprized if ATI pulls a fast one and leaves you
standing at the curb on DX10 for AGP. They have done this kind of crap
before. The main problem is that there likely wont be a next
generation of AGP cards! Everyone seems to be pushing PCI-E, and AGP
is phasing out. So in this case ATIs lack of support could translate
into you having to buy a new computer!
Time will tell, but I would not hold my breath - if past hyjinx are
any indication of things to come from ATI! (Who do they think they
are? MicroSoft???)