There's no actual limit on how large a drive you can use: any bios that supports LBA48 (drives over 128GiB) will support any IDE hard disk you can buy.
There is one limitation, though, which is the maximum size of a normally-formatted FATX drive. Your F or G drive cannot be over 256GiB unless the program you use to format it is smart enough to use 32k clusters instead of 16k - many dashes can't format with 32k clusters, and the only program I know of for sure that does is XBpartitioner. If you use 16k clusters on an F or G drive larger than 256GiB, everything will *seem* to work fine, until you actually use more than 256GiB of space, at which point the drive will become corrupted and you'll lose some or all of your files.
If you have it configured to have both an F and G drive, though (.67 bios), then you won't run into this problem until you have a drive bigger than 384GiB (as that's when the G drive would be over 256). If you make sure whatever utility you use to format the drives is using 32k clusters, you will be fine then
BTW, I use 'GiB' to make it clear that they are in binary: 256GiB is 274,877,906,944 bytes. Most hard drives are sold based on decimal sizes, or a mixture, for marketing reasons: a hard drive manufacturer may well tell you that 256GB is 256,000,000,000, or even 262,144,000,000 (the latter is 1 million KiB, which a few manufacturers confusingly seem to think is a gigabyte). It's quite likely that a drive sold as being 400GB is actually under 384GiB and thus doesn't need 32k clusters if it has both an F and G drive
