There's quite a few things you can do with $1,500. It honestly just depends on where you want to spend it.
Boost will get you the most power for the price though.
I checked your profile, but your car isn't listed there, you should fill it out so we can help you out more.
If you have a 5 speed and you want to make it just a bit more fun to drive, definitely get a short shifter. As far as "power" goes, there's obviously no difference, but it sure makes driving a lot more fun.
Also, unless you're going "N/A", a Short Ram Intake won't really give you any gains at all without tuning and supporting mods. It sounds mean as hell, but I think $180 could be spent better elsewhere.
Granted you're determining whether to spend more money on the car or not based upon the improvements you get from what you spend now, but I still think boost will give you the best bang for your buck.
You could always go Nitrous. Apparently a stock Eco can handle a properly set up nitrous system up to about a 75 wet shot decently well. I'm not personally a fan of nitrous though, mostly because I wouldn't want to have to refill a bottle to go fast. You never have to refill boost (or N/A for that matter).
A nitrous guru could set you straight on the ups and downs of a nitrous system, and how you would want to go about setting up your car, etc.
If you want to go N/A (as PJ says, best reserved for base models, due to their being almost 200 pounds lighter than an LS, an LS will probably be a slug even with a nice NA setup) than I'd suggest a Short Ram Intake, larger throttle body (not too educated on this, you'd have to search what a good size would be. For some reason I keep thinking 58mm-62mm), header, and exhaust. That would free up the flow a bit, and probably even HP Tuners to optimize it all. Here's a small price list of what you could get for a basic start on an N/A application. After every category I'll list the average one would probably be looking to spend.
Short Ram - Could be as low as $30, and about as high as $200. - $180
Throttle Body - No idea on cost >.< $?
Header - $150 - $450, depends on how nuts you want to go. - $300
Exhaust - For an NA application, it once again depends on how nuts you want to go. PJ (DaFlyinSkwirl) has a 3" exhaust, which is ideally what a turbo J would run, but for his mods it works. Supercharged cars generally run 2.25" - 2.5" exhaust. From what I've read, a 2.25"-2.5" exhaust would be good for an N/A application that isn't absolutely insane. (Read: PJ's car) I've seen catback systems go from about $250 - $700. - $500
HP Tuners - Tuning program, Standard is $500, and Professional is $650.
http://www.hptuners.com
That's $1480 without the Throttle Body cost included. Granted the throttle body is arguably an unnecessary piece for somebody who just wants a bit more out of their car. It'll help if you get it, but I'm fairly sure they're a bit on the expensive side.
As stated though, as far as cost/power ratio, boost is probably the easiest/fastest way to go.
Even then, after around 220whp with a turbo or supercharger, I'd start upgrading internals.
It's up to you, and I hope I've helped you out a bit. As far as some good sites to check out for parts, here's a short list. If I forget anybody, sorry!
www.aplusperformance.com
www.gravanatuning.com
www.turbotechracing.com
www.allecotec.com
www.carcustoms.net
I'm not on my home computer so I don't have bookmarks, but like I said, if I forgot anybody sorry. When I get home tonight I'll add the other sites that I have bookmarked if I forgot any.
Good luck, and be sure to post your progress whatever you decide to do.
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