I have a Pc i7 920 machine with 6 gb ram, windows 7 64 bit and nVidia Quadro FX 580.
When I render DV footage with Premiere CS4 (installed on my C drive and write on a RAID 0 D drive) it works at an OK speed (aproximately 1:1 with the project's length) but HD and down converting HD to SD take a lot of time (it took me 1:30 hours to render a 2:50 minutes made as 1080p and rendered as DV PAL). To be fair- the project had several "Dynamic-linked" AE footage in it (that's where the rendering really slowed down).
Is this OK or are there any preferences that I need to toy with to get the GPU to "give more power" to the CPU?
CS4 doesn't really take advantage of the GPU. The Mercury playback/rendering engine is introduced in CS5, so you'll see substantial performance increases there if you have a compatible card. As it stands, your CPU is doing all the heavy lifting.
1.5 hours for three minutes of footage sounds relatively reasonable, but it's highly dependent on what kind of effects you have(including image adjustments), what the AE comps include, and what codec and quality level you're exporting to.
Are you running the Quadro FX 580 or the Quadro FX 5800? The latter is certified by Adobe. If your card isn't on the certified list, if it has Cuda technology and at least a gig of video memory, you may be able to use this hack http://www.indiev.org/?p=308 which is really just sticking the name of the card in one of Premiere's config files... not a brain buster.
I'm using CS5 with a hacked card, the GT240 and I'm squeezing great performance out with multiple video layers and adjustment effects applied.