Tamron AF55-200 F4-5.6 Di II LD Macro lens?

[kats56]

Hi everyone, I am a newbie to digital slr photography having always used film slrs up until last month.
I have just purchased a used Canon 450d with the 18-55 is lens and can't wait for it to arrive.
I want to get another lens to go with the camera and have been reading all the advice on here ( what a fab place for info ) and I like the sound of the Sigma APO 70-300mm F4-5.6 DG Macro Lens and also the prime 50mm 1.8 lens. I am also looking at the Tamron 55-200mm F4-5.6 Di II LD Macro lens which has great reviews and I am watching one at £60 on e-bay.
Would you say these are good choices? Your advice would be appreciated. .
Excuse my ignorance but I know digital is different to film.
Thanks so much for all the great info on here.
Jen