Hi Andrzej,
The camera's recording format is more important in this discussion than 1080i/720p/1080p/whatever.
HDV, XDCAM EX (at least up to 35mbps), and ProRes work respectably well even back to the G5 era. I'm using a dual-core 2.3Ghz G5 and my bottleneck is usually hard drive speed, not processor. So, footage from any of the HDV or SxS Sony cams from the last several years, the JVC HD100/HD200/HD250/HM700/HM100, and the Canon HDV cams will play nicely with your system. AVCHD, though (from the Panasonic HMC150 or HPX300) -- that won't even play or transcode on PPC-based systems, but it should be ok on your system.
Unless you're doing mass amounts of compositing or large multi-cam edits, your computer will be fine. Chris is right -- get at least another 1GB of RAM for FCP. Get 3GB if you plan to use Motion or Color.