What lens have you been using with the 30D?  The problem with 'upgrading' is that each newer and more detailed camera shows up the deficiencies of lenses, processing etc that we never knew were there before.  You've upgraded from the old 300D 6mp to the 'newer' 30D 8mp and it is quite possible that the new camera is simply showing up some problems with whatever lens you were using before.  Very unlikely that the 30D would have all that softness and purple fringing (unless its a faulty camera, but that is unlikely).

You could take the whole thing to Anderson Cameras in Capalaba and for a small fee they'll strip it apart and do an analysis.  I took my Nikon kit lens there because it was 'soft' and they realigned the elements under warranty (but I don't use that lens any more).

Over the past year I have stopped using zoom lenses.  I bought a 35mm F2 cheaply on ebay and nothing can compare to the quality it can produce (for the price).  Perhaps Canon have something similar.  I've just received a 20mm F2.8 also bought cheaply on eBay.  Perhaps you can find something similar for your Canon.

Nobody seems to want to buy these old fixed lenses any more, so the prices are very attractive in many instances.  That's so long as the seller gets real; I am watching for a 105mm F2 which is one of the best lenses Nikon have ever made; the seller wants $850 and he hasn't got a hope in hell.  I've written to offer $450 once he fails to sell and has to relist.

I'm happy carrying around some good quality glass and I don't miss the zoom lenses.

Sharply uses Canon - perhaps he'll chip in with some suggestions for you.

Given that your using the same lenses on the 30D as you did the 300D, I'd say that the increased resolution of the 30D is showing you the flaws in your lenses that the 300D didn't have the power to show.

As far as getting a new lens goes, it depends a lot on how/what you shoot, and how much money you want to spend. For outdoor/daylight and studio stuff (i.e. plenty of light), you don't have to splurge for a fast lens - as long as you can live with always shooting at f/8 and above. If you want to have sharp pics with a shallower depth of field, then you have no choice but to go for something pricey, like a f/2.8 zoom or a couple of faster primes.

I used to shoot a lot of indoor low light stuff (concerts, events, performances, ...) using a D60, 10D, 20D, and 1D MkII, and found that I had to have a 16-35mm f/2.8 for wide shots - the crop factor was just too much. My only complaint with this lens was that it's 2x zoom capability was very limiting and frustrating. It was normal for me to swap this lens on, take a few pics, then swap back on my bigger zoom. I sold this lens after I upgraded to a 1Ds Mk II.

A fantastic general-purpose lens is the 24-70mm f/2.8, but you can save a lot of money by picking up its predecessor - the 28-70mm f/2.8, an almost equally good lens. A good longer zoom might be the 70-200mm f/4 with IS (invaluable), or the f/2.8 with IS if you can afford it.

I used to have stacks and stacks of lenses - primes up the ying-yang - but I sold them all except three: a 15mm fisheye, a 24-70mm f/2.8, and a 70-200mm f/2.8 IS. I sometimes wish I'd kept my 300mm, but what can ya do.

Choosing a lens can be overwhelming at times - there's just so much to choose from. Rest assured, though, that you get what you pay for, and if you decide to spend the extra dollars to get a professional grade item, you will not ever be disappointed with your purchase. Also, you should seriously consider buying used lenses. I purchased most of mine using eBay, and have never been disappointed.