As for ATI, I haven't heard any major hardware updates required to work with even their highest-end chipsets. Is that correct?
It depends on your current hardware set-up. It always does.
If, for example, you're wanting to upgrade to an SLI (multi-graphics card) set-up. you have a blazing processor (core 2 duo E6700), and a good power supply (700 watt), but only have one PCI express slot in your Mobo. You can get away with just upgrading your mobo quite easily. as well as buying the pair of graphics cards of course.
But if you're running older hardware - a pentium 4 2Ghz, 300watt power supply etc. You'll need to upgrade practically everything if you want to run a DX10 card. (except maybe your case)
As it stands, nVidia do have higher performing cards. But for more money. ATI offer better value I would say.
They're also more advanced I would say. ATi has been working on unified shaders for years longer than nVidia have. They did the Xenos in the Xbox 360 back in 2005.
Crossfire solutions are also more than a match for SLI based set-ups. Unfortunately, you need a different motherboard and chipset for each.
DX10 has only just started though. Besides, with all the consoles on Dx9.5 spec hardware or worse, we won't see widespread utilisation of it until around 2011.