This is just a guess... feel free to disagree and enlighten
But my guess would be whatever is in the Xbox 2, which is rumored to be released at the end of 2005, will be the most "significant technological advance in computer graphics technology".
Why? While the Xbox2 is not a PC necessarily, it will do a couple things that affect PCs:
1) Beyond developers trying to keep the lowest common denominator in view, I think the consoles tend to hold the PC market back some in regards to games' graphics. On the flip side I also think when new consoles are released it is a new "minimum" standard that is completed. Based on the outdated Xbox2 specs, it looks like the new consoles will rival high end video cards. My guess would be over the next 24 months we see more and more PC games shifting away from DX7 as a baseline (e.g. look how D3 and HL2 run fairly well on older HW) and begin hitting DX9 as the basic featureset.
2) The Xbox2 will be introducing a unified shader architecture. How this affects the PC market (good or bad), I do not know, but with WGF and R600 coming down the pipe and Nvidia looking at a non-unified structure from the HW point of view, this will definately be a significant advance. I am curious to see how this type of architecture affects content creation, because theoretically being able to dedicated all your Vertex Shader tasks should help out 3D workstation apps.
3) If the rumors are correct, there will be a "XBox2 PC". Imagine millions of PCs with high-mid range video cards in it. That is a nice lowest common denominator! Forget FX 5200s and Radeon 9200s!
Just my opinion... but then again R520 and NV50 could totally blow us away, or something totally different could. Who knows... shall be exciting!