Personally, I haven't seen much difference in performance between 98 and 98SE. There are but a few features that are different (ICS, IE version, etc). The advantage with starting with 98SE is that it's 98 patched. There are less updates to download from Windows Update site, and there are some not available for the original version of 98.
When considering the upgrade, don't worry so much about the difference between 98 and 98SE. Instead, I'd be more concerned about the overhead you gain going from 95 to 98. There isn't much, but enough to notice a slight difference on a slower PC.
I recall running 98SE on a Pentium 133 with no problems...
~cdogg
"The secret to creativity is knowing how to hide your sources."
- A. Einstein