Please, don't suggest the Mini

I use my comp for everything, but manily graphic design, but I want to be able to use GarageBand and do lower end cideo editing (clips for the web)

I have read that the original ATA-66 controller that shipped with my comp is a bottleneck to performance and that a new faster PCI controller card should be an essential upgrade. That true?

ATA-66 is slow compared to ATA-100 or even SATA.
But you can only feel it, if you use application that uses disk intensively.  If you use Photoshop to do 100MB file, you can feel it, as Photoshop uses harddisk for its swap files.  If your system is low on RAM, such as only 128MB and running OSX, then you can feel it, as OSX uses HardDisk for its swap.

Imo, you'll have a better experience with GarageBand if you use faster processor.  Your system (cpu and memory wise) is adequate enough for a low end video editing (using iMovie, or even FCP).

For comparison, I'm running OSX 10.3 with my Cube (G4-450), with 512MB RAM, with internal 20GB 5400rpm ATA/66.  Using it for simple video editing using FCP, Photoshop pictures with average size of 30-40MB, Dreamweaver them into web pages, Office X rarely, Firefox and Mail.app daily.  Sluggish? not really.  Slower than if I run OS9, I can't remember, since I've been using my Cube with OSX since OSX 10.1.   (1 film scanner, 1 desktop scanner, 1 external fw cd-rw, 1 iSight are attached to this computer, so this is a very busy machine!! :))

The only upgrade I installed in my cube is 2 200GB External FW Harddisk, and 1 400GB External FW Harddisk (to store the images I capture from FCP).

I feel somewhat sluggish perfomance of GarageBand running on my Cube.

my 2 cents

You use a dual 1.8 Ghz G5 with 4 GB of Ram and it still is not fast enough??  What specifically are you doing?  We have a number of systems and our newest is a Dual G5 1.8 with only 1 GB of Ram - we are able to use Final Cut Pro, Soundtrack, all of iLife '05 apps without any problems what so ever - in fact the only problem we are having is finding enough hard drive space for the capture scratches.

Conversely, I just purchased a Mac Mini for myself - and it chokes on iLife '05.  When I try using Garageband the system even locks up to the point I have to reboot.   Granted I only have 256MB of memory in it - but consider that we have an iMac G4 at school with same ram, cpu and video - and it runs iLife '05 very well.  The difference is the underlying system (frontside bus, hard drive speed, etc).

I think the real bottleneck is the hard drive speeds are not fast enough for the system speeds - I have seen a G5 machine crap out if it cannot get to the hard drive the way it wants .. hopefully these issues will be worked out in Tiger.

I already stated it, but I will state it again - since you are wanting to "... feel the crisp, sprightly, OS9-like performance in X ..." I would recommend a new system.  A PowerMac G5 - with an ATI Radeon 9600XT or higher and 512MB of ram would do you perfect.

Hmmm.. This could go on and on.
Your Digital Audio G4 is very upgradable.
Yes Processor Upgrades to Dual for supported apps, SATA or PATA and SCSI upgrades available too, as well as graphics cards......, Memory is another factor ...What do you want to do?
Perhaps the MINI is what you need...lol
Dave