Since your budget is the restricting factor, here are some Canon lenses that fall below the S$800 price-tag, plus some suggestions:
Do note that I'm a Canon nut.. So I don't know anything much about 3rd-party lenses like Sigma and Tamron. I hear they're quite good though, but I still won't buy them.
-----
Canon EF-S 17-85mm f/4.0-5.6 IS USM : RS$1,229 - CP $860 / MS $860 /AP $1,020 / CP-M $880 / OP $750 (without box)
The Digital Picture Review
The Kit II of the Canon 400D, this is an alright lens with a good range and quite good image quality. Drawbacks will probably be the high amount of barrel distortion at the wide end. It's recommended for a general walkaround lens.
-----
Canon EF-S 55-250mm f/4-5.6 IS : CP-M $410 / OP $360 / MS $399 / AP-S $360
The Digital Picture Link (No review yet, just go check out how it looks like xD)
Hmm.. I have mixed feelings about this. Image quality isn't very good. It has a good zoom range, but 55mm for a walkaround is a bit too tight for some shots. It's the best bargain if you need a high zoom range.
-----
Note for the above 2 lenses: They are EF-S lenses, which mean that they can't be used on full frame bodies (eg 5D / 1D). Meaning that if you ever take a huge leap and upgrade to the better bodies, you can't use these lenses. But since you're just about to buy your 400D, I don't think you should be bothered by it too much.
-----
Canon EF 55-200mm f/4.5-5.6 II USM : RS$490 - MS $380
The Digital Picture Review
Check out the above review. Apparently it's not quite good a lens, so I wouldn't recommend it.
------
Canon EF 28-105mm f/3.5-4.5 II USM : RS$690 - MS $338 / TCW $450 / CP $348
The Digital Picture Review
No idea about this lens. Apparently it's a good build and decent price-for-quality lens. Don't bother about the slightly cheaper f/4 - 5.6 version.
-----
Canon EF 28-135mm f/3.5-5.6 IS USM : RS$1,100 - CP-M $690 / MS $665
The Digital Picture Review
Highly recommended from me. I borrowed it from my friend to shoot for a week or so, and it performed pretty well. It won't perform well for low-light no-flash photography, but is still a very good lens. The IS helps a lot for this zoom lens with a small aperture at max zoom.
**********
Canon EF 50mm f/1.8 II : RS$160 - / MS $110 / AP $125 / TK $135 / JO $130 (w/ gst) / TCW $135 (grey) / CP $125 / AP-S $130 / OP $125 / MS $120
The Digital Picture Review
A highly recommended and cheap prime lens. It is the cheapest of all lenses going at around S$130, but it's a good choice for your first few lenses. It's a prime, so it doesn't have the zoom-capability ie you'll have to move around to compose your shots. I own one, and I use it more often than my other more expensive lens.
**********
I recommend the 28-135mm f/3.5 - 5.6 IS USM Zoom and 50mm f/1.8 II Prime. I'll just keep recommending the cheap 50mm f/1.8 to anyone cos it's such a bang-for-buck prime lens. That'll make the hole of your pocket about S$775 from MS prices.
Hope this helps =)