It depends where you are. If you're in a block of flats, then others can hack your WiFi if they are so minded. If you're "detached", it's likely that the neighbours won't have enough signal to hack you - especially if you put the Wireless Access Point in the middle of the house.
Try searching for other wireless connections. If you don't see any, it's a fair bet (not a cert) that nobody else is in range. If there is a determined hacker sitting in his car outside (or in the house next door) then nothing is going to stop him - apart from not using wireless access.
I normally use hard-wired for the "fixed" machines, and wireless for the laptop and the iPaq. I don't notice any speed difference unless I'm downloading large batches of stuff.
I just ran the ADSLguide test twice using hard-wired and twice using wireless, all on the laptop, and I got 5.6meg down/369k up both times on hard-wired, and 3.8meg down/369k up both times on wireless.
Given that the wireless is running at 54 meg, there's no good reason why wireless should be slower than hard-wired. I suspect there are settings I need to tweak, but 3.8 meg is faster than I can type
If you're only getting 400k or thereabouts on your downlink, are you with the right ISP? Or are you in deep rural countryside where ADSL is stretching the limits?
[Edit: I pay £15 a month for this "up to 8 meg" connection in rural Norfolk. The exchange just happens to be a couple of hundred yards away. In Essex, I pay for 1 meg, they give me 2 meg because they messed me about badly a while ago, and I actually get about 1.7 meg.]