My assumption is (I'm yet to have hands-on-experience with eSATA), that PCI cards like
these are good if you don't have any SATA ports left on your motherboard (The white and black thingys on your motherboard). Although looking at some reviews the cheap ones aren't crash-hot so it might be wise to steer clear. If you do decide to get one you'll obviously need a spare PCI slot to put the card in.
Probably more wise if you're not using your SATA holes at the moment is
the second link I posted. They appear to just connect from the Motherboard ports to a bracket that's easy to install. Although, it will only support 'hotswapping' (That is taking the HD in and out while turned on, like a Flashdrive) if your motherboard supports it natively, which is hard to tell unless you know the make of your motherboard. But at $3.19 it's not going to kill you if it doesn't work I guess.
Another thing to consider is buying a
firewire PCI card, if your computer doesn't already have firewire. It's better than USB2, and has less problems than low-cost eSATA cards appears to have at the moment. Another advantage is you might not have to use a powercable, depending on the drive, whereas with eSATA you will have to plug into a powerpoint. Though I don't think you'll find a 350GB Firewire HD in your price range.
If you go down the eSATA route,
this doesn't look like a bad buy. But remember if you get a purely eSATA drive, you won't be able to just unplug it and take it around to whereever, as it most likely won't have a eSATA port. So personally I'd probably for out the extra 50$ for the
500GB WD, and buy the $3 eSATA bracket for home use.