Ok then you do have the Dual-Processor model and not the Dual-Core one. That's too bad because DDR2 is way more cheaper then DDR.
If all you need is 4gb and don't want to upgrade later, you can find 512mb modules for kinda cheap so just buy 8 of those. Otherwise I'd just buy 4x 1gb and that will leave you with 4 open slots to upgrade in the future.
Check ebay or even OWC or Crucial for current prices.
Make sure you buy low density memory. I haven't tried high density before and I'm sure it even says that it isn't compatible. All the cheap prices on ebay are pretty much high density. Also check craigslist.com for local sales. I bought 1gb(2x512) for $10 and recently resold it with a 2gb kit (4x512) for $50 so you can see the highs and lows. I got 2gb (2x1gb) at frys.com for about $18 but that's after a rebate. The promotion already expired though. Expect to pay around $20-30 for a good quality name brand 1gb module.
Yeah, I have two high quality and warrantied 1 gig chips that are guarantted for Powermac g5's dual processors like yours. PC 3200 ram. Best I could do would be 60 shipped since I paid 80 shipped. They are unused and brand new. I would trade you for your 2 gig chips for the Powermac Dual cores.