This is my wish list for you;
1. cpu - a dual 7448 upgrade
2. ethernet card - Gigabit Ethernet Card - http://eshop.macsales.com/item/Sonnet%20Technology/GE1000LA/
3. bootable SATA card - http://eshop.macsales.com/item/Sonnet%20Technology/TSATA/
And then you can upgrade to Leopard.
All great recommendations but if running 10.5.2 or higher there is a much cheaper PCI gigabit ethernet option.
I made this thread a while back about a Trendnet gigabit card that works plug and play on 10.5.2+. I bought two on ebay for 12 each.