Two thing I would add to this...
I've never seen this 'artifact' probelm using PSD files. It may simply be an aliasing thing I wonder. I.e. the jopin between a blue object over a red object may result (if anti aliasing is in effect) with a few pixels of mixed colour - hence purple.
Secondly. Absolutely do NOT buy Premier Pro2 and After Effects 7 SEPERATELY. Really really do NOT do this. If you have then cancel the order NOW!!!! I'll explain.
AE retails individually at around £1000 UKP. and Premier Pro 2 retail at around £700 UKP.
For £1600 UKP you can get the Production studio. This gives you both Ae7 and PPro 2 along with Illustrator CS2, Phtosohop CS2, Encore DVD 2 and Audition 2. And, only for Production studio users, you get 'Dynamic Link' as well which is a workflow feature between apps allowing you to cut and past between them requiring no re-renders. i.e. edit a clip from you PPro timeline in after effects and updates are instantaneously in the PPro project timeline with no export/import or re-render. It's great.
So, my point is I'm not telling you not to get those products but to save money and get more.
No doubt you're getting the PS already and I've typed all this for nothing.