A PSU (power supply unit) for AMD64 should have a 4-pin 12v connector (also called P4 connector, some PSUs have a 6pin connector which is compatible with the 4pin version) and have enough wattage to power all system components. For a PC with a small graphics card, one HDD and one CDROM, a 300W PSU should be fine.
AMD64 is 64bit. K6 is 32bit. K6 cannot run 64bit programs, but AMD64 can run 32bit software. there are both 32bit and 64bit versions of most major GNU/Linux distributions. SuSE sells SuSE Linux 9 Professional for 90 Euro which includes both the 32bit and the 64bit version. Another Linux distribution with both 32bit and 64bit support and free download is Debian.
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