I think building a new PC would be an excellent Christmas gift as it'll be a gift you will use for the next few years, plus the knowledge and learning you will experience will last a lifetime.
I agree with the previous poster- if you focus on getting quality, high-end components yet skimp a bit on the videocard, this would be the best option. That way, when DirectX 10 comes out, you'll still have a decent/moderate videocard and upgrading next year will be less painful financially.
What kind of budget are you looking at? I'd say build a really nice Core 2 Duo system with dual-channel DDR2 memory and say a middle-range 3d card (like a 7800GT or even 7600GT/X1650 Pro). You'll have good game performance, medium to high settings, and save 100-200 on the videocard rather than "cutting edge" that will be obsolete in 5 months to a year.