The mac will read all PC files like .jpg .doc .xls, but not programmes (.exe). This is irrespective of whether they are on another networked computer or an external hard-drive. (or a 20 year old floppy, if you buy a floppy drive). Depending on the software bundled you may need to get MacLinkPlus to translate files from PC only programs to mac equivalent. If you have any trouble, PM me I have 20 years mac experience; there is also the apple on-line knowledge base and it is quite difficult to find any problem that cannot be solved there; don't worry its far easier than swapping other way round. There are also PC emulation progs, but they are expensive and slow.

On reflection, I think you are making a false assumption if you think you need the external drive to transfer your existing files. (a) it is not necessary as the mac will copy across all or anything there is on the PC by networking the 2 together; easy as the mac software detects what there is at the other end of the cable or the airport; at worst you may have to put everything into 1 folder on the PC and assign a pass-code to it (b) for long term back-up it will be better to have the external HD formatted for mac (or rather just not formatted for PC). This is because the mac operating system includes a very powerful database in its structure, so all files are stored with a wealth of information that enables you to find them later if you (or nowadays the computer itself) has been less than intelligent as to where they should be stored.

But the HD can be bought shortly after the box, not needed concurrently as you still have the PC as back-up. 2 of us now are recommending LaCie as reliable. If you go for a cheapo, there are a certain number of know failures around and the suppliers rely on you having a guarantee (=insurance policy) to get a replacement. Also when a really mac compatible drive is about to fail (ie has a fault in 1 of several billion sectors), the mac OS locks you out of messing things things up any further. At that stage, copying to a working mac is still possible by, as it were, sucking from the mac end using the UNIX machine system. That also applies to the internal drive.