About overclocking, the 5900xt can go really far without heating much nor being unstable (****load of crashing). It really depends on the 5900xt. There are two kinds of 5900xt; ones with 2.8ns memory or 2.2ns memory. If you can get the 5900xt with 2.2ns, then it is possible to overclock it to reach 5950U speeds (475/950). You also have to flash the bios for better voltage intake so stability and fan speed will increase. Here's more details into it; http://www.xbitlabs.com/articles/video/display/faq.html
I wouldn't bother with the 5900xt though it's a waste of time to do such procedures to make it work perfectly. Also, if you mess up, it's over for your 5900xt.
The 9800 Pro might have reach it's limits for a .15u (380/680), but with that huge heatsink & fan, it is possible to overclock it to match a 9800xt (might not be stable though). Even better if you can get a R360 core instead of a R350 (which is quite rare).