Too late pal.
Initial investment for the body only should be about 50% of what you're willing to spend.
If u didn't realise that before, means U've caught the buy buy buy bug.
No cure but to sell away the 300D.
The thing about buying a DSLR is that you're also willing to spend some more $$$ for memory cards and of course, lenses and perhaps other accessories like an external flash and batteries. These can be shocking hidden costs (>$1000 at least) for the newbie jumping into the DSLR arena.
Canon did well to market the 300D for amateur photographers but you definitely need to consider lens purchase along the way if you still wish to stick with the 300D for some time. If not, use the kit lens well and try to save up soon for at least another lens. Else, it is not quite justified to spend so much on the body but too little thought on the lens.
I'm sure there are many 3rd party Canon-mount lenses you can consider for the 300D. Maybe if you state the lenses you have in mind, many here can help you. I'm a Canon DSLR user too... but only use Canon lenses.
Sorry I didn't quite answer your questions fully but thot I'd offer my humble advice...
You already have a wide angle with the kit lens. For telephoto you could consider the Sigma 70-300 f4-5.6 APO Macro Super II or Tamron's offering with macro. Sigma one costs ~$340, while tamron one is less than $300.
Thanks kiwi, i know additional lens would be high hidden cost especially Canon Lens. That's why i am asking if other 3rd party lens e.g. Sigma lens are wisely and good for cheaper value. And any infor on other lens price range?
From $135 for the cheap-and-good Canon EF 50mm f/1.8 prime lens to $3000+ for a 70-200mm f/2.8L IS USM lens...
For 3rd party lenses you can probably mark it down 20%-30% off the genuine Canon lenses but because they don't always match the Canon counterparts feature-to-feature, it's difficult to make comparisons.
The sky's the limit. You've got to carefully consider what kind of photos you want to shoot and then narrow down your lens range.
1. Knowing what you want from the start can save you a lot of money and heartache. Most of us waste our money in changing our minds. Some of us even change our minds on the entire system (eg switching brands).
2. Once you know what you want, wait for it to surface in the used market. Then pounce on it.
3. There are some third party lenses that are very good - like the Tamron 90/2.8 macro - but almost all 3rd parties do not hold their resale value as well as the Canon originals.
4. You CAN tell the quality diff between a consumer lens and an L lens on a 300D at 100% magnification. But you won't be able to tell the diff in prints unless your prints are bigger than A3. If you're doing 4Rs with the occasional 8R, and web publishing, can give the Ls a miss for a few years first.
5. Never say "That's the last lens i'm getting. Now i'm done." The inventory grows to fit the budget.
Hah! This is the sting of going SLR! Be prepared to spend in the region of $250 to $2000 for additional add-on lenses from now on. ;p
Any EF mount lens from Sigma, Tamron or Tokina should be physically compatible with the 300D. However, some of the older 3rd party lenses may not be electronically compatible, and have to be re-chipped to work with this cam. So try before you buy.
Seriously, the kit lens is of pretty decent quality and is fairly wide so you probably don't require another lens <=18mm. Personally, I also use the Tamron 28-75/2.8 XR Di (~$600+) for mid-range and recently got a used Sigma 70-200/2.8 EX HSM (~$800+) for longer tele. All 3rd party lenses as you can see. I also now have a very new and seldom used Sigma 70-300 APO2 which I might be willing to sell .
You can try perusing the DPreview Canon SLR lens forum to get some idea of what's hot and what's not.