The only reason you might want a Decoder card is for TV-output.
The TV-output on a Hollywood plus is much better than the Voodoo3s and GeForces I've seen. The Rage Theatre chip on the Radeon is close...but since you don't have a Radeon it's moot.
If you just want to watch on your monitor go software. You need about a 400Mhz CPU and a video card with good 2D output.
Various video cards have various hardware assist functions, but as long as you have a fast enough CPU you can run in pure software mode and then the only thing on the Video card that matters is 2D quality.
Ed: on a monitor hardware vs software is very close in display quality.
I would recommend WinDVD 2000, with a runner up of PowerDVD 3.
WinDVD has 2 key advantages in my mind:
[*]Nicer laid out right click menus, which you use for control in full screen mode
[*]Downmixes LFE channel so you get all the bass effects in 2 or 4 speaker mode.
If you are using 5.1 analog mode or SPDIF passthrough to an external decoder the sound one doesn't really matter.
Another benefit of WinDVD is that it supports the hardware playback functions of the Radeon, but we already agreed you don't have one, so that's moot to
PowerDVD 3 does have a nicer interface panel, but I always find myself using right click menus for controls (play,stop,puase,etc), and I find the right click menus are more nicely laid out in WinDVD.