Is Sony a gimick? - abolutely not - they are excellent cameras and Sony have an ever growing share of the DSLR market (third behind Canon & Nikon), they will be around for a long time to come. Also the use of the Minolta lens mount means that there are a lot of good used lenses out there.
Are my lenses scrap? - no they sould work, the Canon will but the Sigma may not (some older Sigma's don't work fully with digital bodies) take it the shop with you and try it out.
Are Canon worth the extra money? - only you can decide... to be honest I'm not sure.
I the 450d £100 better than the 1000d? - I think so, the 1000D is basically a stripped down version of the 450D, I'd definitely pay the extra for the 450D.
I agree with the others that you should try them and see which you like best, though if you do go for a Canon you'll save some money on lenses. The Sony A200 is a great deal for them money, the twin lens kit is worth a look too -
http://www.warehouseexpress.com/prod...px?sku=1024309