Hi All,
First off, I'm new to the D/SLR world and require some advice

My dad had an interest in SLR photography for many many years, the number of Amateur Photographers in my mum's Garage is testament to this, but I was never that interested and always thought why does he have so much camera equipment although from what I've read on here its the curse of photography.
I've had a compact digital camera for quite some time and find it ok for the quick snapshot but it's pretty rudimentary and the PQ isn't great so I've decided to invest in a DSLR, in fact I decided to do this a few months ago but wanted to investigate what's available before making my purchase as it will be a long term investment for me.
This camera will be used mainly for general use, and I have an interest in taking pictures of lit buildings and general engineering (bridges etc..). I also go to motor sport events occasionally so a capable camera for this would be useful also. Lastly, I can see me taking landscape pictures on holiday with the occasional nature shot. Another point to consider is that I have windows Vista 64bit and so will need something compatible with that, although I believe most cameras will be fine.
I have a few cameras on my final short-list, Canon 400D, Canon 350D, Sony A100, and a Pentax K100D. My budget is about £800 but I will spend a little more if needed, although less is better! I'm in the middle of decorating (oh joy!) and will be looking to make my purchase after I'm finished at the end of next week.
I have to admit I was all set to buy the Pentax with the kit lens as it seems great value and would leave me a few hundred pounds to spend on lenses, SD memory and a bag but the more I think about it the better off I feel I'd be buying a better body as the more expensive cameras above aren't much more (£60ish extra). Also, no anti dust mechanism and only 6mp is also making me hold back although I'm not sure if they are really important.
Out of the above models I like the Sony the best on first impressions although I'm concerned about Sony's commitment to the DSLR scene, and the availability of future equipment (lenses etc..), also, from what I can tell there equipment seems more expensive than Canon or Pentax equivalents.
It would be much appreciated if I can get some advice on what to buy, lenses to purchase and anything else I'll need to get including memory and bag.
I realise there are a lot of similar threads on here and I've read most of them! but I guess everyone's different, apologies for another one none the less!!
Thanks