Unfortunately, Yes. But how you use the system will have the greatest bearing on the degree you're affected.
That is, there's already limitations for graphics cards, and Apple's going to drop support for EFI32 in OS X in the near future. No one's certain as to the exact version will drop K32, but historically, Apple doesn't stick with interim products for very long, and 10.7 has been developing rather slowly (best case, K32 gets dropped with 10.8).
So if you'll be fine with say SL and a 5780 (worst case) long enough to hold you over until the next system, you'd be fine. But if you're going to need to be able to use newer versions (K64 exclusive) of OS X, and have a graphics requirement that forces you to upgrade, then it's probably not the wisest expenditure (better to buy a newer system now).
Assuming you stick with 2D work, you'd be fine. But if you want to get into 3D work, you're going to need more system to handle it (especially on the graphics aspect). I've no idea on your future plans.
The really odd thing is (more like disgusting actually), you'd be fine with both future versions of Windows and PC graphics cards if you dedicate the system as a Windows box (Linux as well).