Don't spend any money on things that you're pulling out.
If you really are going to swap, then live with the power that you're making now. Worry about making more power with the swap drivetrain. Never upgrade stuff that you're already planning to replace. It's like throwing a new CPU into a computer where you're already planning a new motherboard for.
That said, there probably are a few things that you can do which will help.
Common parts for exhaust. A cat-back exhaust system suitable for the swap drivetrain can get you a couple of ponies. Don't expect much, this is practically never much actual power by itself.
Lose some weight. Look for opportunities to remove a couple of pounds from the chassis. This isn't "more power" per se, but will certainly improve the 0-60.
Take a look at your suspension. If your engine has 258,xxx miles, then chances are your suspension (Or at least large portions of it) does too. Check out bushings, subframes, and major components.
Other than that, change the oil, and get a new air filter.
