Dont count on the 747-200 going away anytime soon, most will certainly still be around by 2006, thats not too far away. The supply of new build 747s is quite low right now so there is no reasonable subsitute yet with a capacity that can meet the 747s. If you look at the entire fleet of 747s, -100s,200s,300s,400s just a tad under 90% of them are still flying. That 10% includes those that have been scrapped, written off (crashed) and are in storage. The only direct replacement for the 747-200 freighter is the 747-400F there are precious few of those compared to -100 and 200s that were either built new or converted to freighters.
The MD-11F is the only other freighter than can compare to the 747 freighter although as we know it is still not as large. It can however operate much more efficiently over similar stage lengths and was regarded as a likely candidate to replace ageing 747Fs. But as we know that program is also dead so they only other widebody freighter being built is the Airbus A300-600 and if we can call the 767 a widebody it can also be counted but both the Airbus and the 767 are roughly half the size of a 747.
I bet we see 747-200s flying around for 20+ more years, at least until either they become too innefficient to operate, suffer some kind of fatigue that older aircraft sometimes have or a replacement of similar size is introduced- perhaps a 777-200F?
Remember there are a lot of 727-100s/200s still out there flying around, the oldest of which predate the oldest 747-100s by 6-7 years, not to mention all the DC-9 10s,15s,30s and 40s still out there. Old airplanes dont go away they just get converted to freighters!