Older hardware, big files, poor performance

I've been trying to get kdenlive to work with my system but I'm having some problems with playback.

I'm using some 8Mb/sec mpeg video files - NTSC 4x3 - with 48khz wav files for sound, and my playback is sluggish - a minute clip takes a minute and 10 seconds to play.

For hardware I've got a 1.2 Ghz AMD Athlon processor with 256 Meg of DDR RAM. Storage is 185 GB Seagate 7200 RPM drive, a an Nvidia 5200 FX video card. Software is Kdenlive 0.4 on Debian Etch with the Nvidia drivers. Both Kdenlive and the drivers were installed the 'Debian' way via Synaptic.

I have a slightly older and slower machine: 1 Ghz Duron with 512 Megs of SDRAM and an Nvidia 440 video card and the same software set up, and it's just as slow there too.

The card does have acceleration turned on.

I've checked the file using Avidemux and it's sluggish there also. When I switch window managers from Gnome to TWM the clip in Avidemux plays much better but I'm still getting the same sluggishness in Kdenlive.

I know I need more RAM and possibly a CPU upgrade, if I can find the right CPU for my motherboard. A new system is *not* in the cards right now. Is there anything else I can do to improve either one of my systems? I'm thinking of installing Sid on the older machine and getting the latest kdenlive - since it's a spare system I'm not afraid to break it.

Thanks,
Mystery Dance