Since the G5 machine is perfect for your needs and the software is paid for, my advice is to hunt for a new/used video card in the 500 MB or better range or buy a used G5 and use that one for parts.
More performance isn´t going to help you if you don´t need it. The PM G5´s have a whopping 1Ghz bus PER processor and they are separate processors, not the dual cores sharing a bus like today. So the machine has plenty of bandwidth, meaning it can access drives, video cards etc very fast. Data can get in an out of the processors pretty quickly and heat isn´t a issue like it is with the newer multi-core processors. The key to getting a G5 duallie fast is faster hard drives and video cards and decent memory. 7,200 RPMś drives (or faster if you dare) or 500 MB or better video cards, enough RAM to meet all your always running needs.
I had two 3D games, Unreal Tournament and Doom, running each in their own window at the same time on my Duallie G5, with a better than 60fps per game. Or I could have 3 HD (1080i) TV channels and two SD channels on at once, plus a mix of other things going.
The all-in-one iMac´s have their drawbacks, first off you can´t access the hard drive, memory or video card. If one part goes down the whole machine is out of commission. And they get hot due to their closed design, resulting in the processors being throttled to keep the temperature down like in laptops.
The new iMac´s all have that annoying reflective screens which could be a issue if you like normally bright rooms. With the G5 you can choose the monitor you want to use.
Having owned a G5 dually myself, sometimes just getting a new monitor, keyboard/mouse makes one feel like they got a whole new machine, without the huge cost.
If you decide to save some money and just install a new/used video card yourself remember these few things:
1: You have to install the drivers for the new video card (download from the card makers site or have the disk) before making the hardware switch.
2: Remove the power cord and discharge static electricity before opening up the G5.
3: Don´t touch the components, only the plastic edges of the video card.
4: You´ll need some small magnetic tools, screwdriver, for the removal/installation.
5: While your in there, use a can of compressed air and lightly blow out the dust and wipe the fans/inside with a static free cloth
Also since you´ve installed a new video card, the drivers for the card will most likely not be on the G5´s OS X installation disk that came with the machine so it won´t work to boot from. To be able to boot your G5 from something other than your boot drive (in case it fails) you will need to clone your whole boot drive to another internal/external drive.
Rather easy to do.
1: Disk Utility Erase with Zero a new blank drive to map off any potential bad sectors (only need to once per drive, will erase all data) equal to or larger than the boot drive.
2: Download Carbon Copy Cloner and clone the whole boot drive over to the other, Disk Utility Repair Permissions on both afterwards.
3: Hold Option and reboot, select the clone and test it out. Simple.
If you got BootCamp itś a bit more involved.http://theappleblog.com/2010/01/12/h...-to-a-new-mac/
If you got Filevault enabled, ask over at Carbon Copy Cloner forums if it will boot a cloned Filevaulted image.
If I was you, I would get me a new, fast boot drive, a newer video card and perhaps some more RAM, likely all for under $300. Feel like you got a new machine.
Also upgrade to Leopard
Should be able to buy the disks online or eBay, check compatibility with your current software, what you will have to upgrade and it´s costs if any. Clone your boot drive first and check it out, disconnect, then upgrade Tiger to Leopard with the disk.
If this is all too much for you, I´m sure you could find a local Mac friendly computer support that can get your machine right and fast for you, it has the potential believe me!
I´d do it for you, but your far from me.