I can barely afford what I have. The X1800GTO CAN'T be used in an SLi configuration. The 7600GS can always be upgraded to a dual-card configuration at a later date. That's why I invested in a dual-PCIe motherboard. And don't even try with the core 2. I've seen the price. They're about $200 and more!
You could use an X1800GTO in Crossfire, but you need a Crossfire motherboard and a master card.
And about the previous poster mentioning P4 and Celeron - the Celeron isn't great.
I've found that every Celeron I've used has been awful, no reflection on your experience but I find them to be very weak processors.
No competition for the Athlon.
A P4 could overclock well, but AMD chips tend to overclock better and run cooler, and are generally faster as well.