If i'm using html 4.01 transitional, will I run into problems if i use
<br /> as the break tag instead of
You will not have any display or rendering problems at all, but your page will not validate. Validation checks the syntax used in a page against the syntax required by the stated doctype. If you use validation as a way of finding errors or trouble-shooting, this will get in the way.
But Dan Wellman's question above is important. Why did you choose the doctype you did? For that matter why do you want to add the trailing slash to the
br element? These two choices are in conflict with each other and hint at a lack of planning or at least a lack of familiarity with the different doctypes and their reasons for existence.
No. HTML 4 void elements (br, img, input, etc.) do not have the trailing slash. That said, it won't affect the rendering of your page if you use the trailing slash - you just lose a few nerd points.