You need at least 256MB of RAM with WXP, especially when you have shared memory. Your system will pull at least 110 MB of RAM after a clean reboot.
Burning an ISO file at 16X will use up a lot of CPU load. My 600MHz PIII system can deliver a min 29MB/sec at the hard drive with no other application competing for the CPU clock time. However, when writing at 16X, the buffer will occasionally go to zero due to >97% CPU load. When the CPU load hits 100%, then the transfer speed from the hard drive will decrease, which MAY result in buffer overrun. I have no problem burning at 12X.
I'm not sure if the AMD Duron 950 is much faster than the 600 PIII. The AMD should be able to write at 8X if the HD can output at least 25 MB/sec. A full burn at 12X will take 7 min. Knock off another minute @ 16X. If your burn time is longer with quality DVD media, then your DVD burner is probably slowing down due to insufficient data flow from the hard drive at high CPU load.