For backup if something goes wrong, you should always have a clean 1.5 handy. I have a clean BI install, clean 1.6 BI, clean 1.2 RTW, clean 1.5 RTW, in addition to 8 different versions of EB (although I could probably clean out 5 of them as they are 0.8x installations and some intermediate versions between 0.81aV2 and 1.0).

Using hard disk space will not affect performance, unless it causes the disk to become fragmented due to low free space. A simple defrag will fix that.