I'm in the same boat. I have a 17" 1.83MHz CD which, like yourself, I am not happy about being 32-bit only...especially with Snow Leopard coming out soon.
My dilemma is if it's worth it. The T7400 goes for ~$150 on eBay depending on where it is (US vs China). That is far cheaper than it used to be (sold for $428 retail on 'egg). But granted if the swap does work, I believe these machines are still limited to the 2GB RAM? Someone correct me if I'm wrong. My other C2D white model only recognizes/uses 3GB even though I have a matched pair of 4GB total. I suppose 2GB is ok...but would be nice to have more with better 64-bit support coming.
Also, while you've go this thing open, an HDD swap is probably a good idea. The 160GB OEM isn't cutting it anymore in terms of capacity.
All in all is ~$250 (CPU + HDD) worth it? Passmark puts the T2400 at 868, T7400 at 1218;...a 40% increase. The E8335 2.66GHz chips in the newest iMacs is scored at 1823; 110% and 49% faster than the previous two chips. And this is just CPU performance...let's not forget the newer iMacs have faster GPUs and bus speeds. So I suppose a T7400 (or T7600) wouldn't be too bad for that little investment if a 17" screen doesn't bother you (can be expanded/spanned w/2nd monitor).