It's price vs. performance here
*goes to configure the Dell machines*
*realizes he's in the australian Dell site and that's why he's seeing base configs at almost $3000*
Base config (P-M 725, 512MB RAM, 40GB HDD, DVD/CD-RW combo) price is $1,903. That's with the 9600 128MB.
7422GX is $1399 +tax , with AMD64 3400+, 1GB RAM, 80GB HDD, DL DVD+/-RW, and Radeon 9600 64MB RAM.
The 8600 may give you a little more gaming performance (128MB vs. 64MB, a little increase but not much), but the massive extra price would not be worth it.