OK.. the specs for the Dimension 4300 are on this page..
http://support.euro.dell.com/support/edocs/systems/dim4300/specs.htm#1101572
To save you reading it... it simply states you have some USB ports... not
the version (i.e. speed).
Since it's a fairly old motherboard, and the version isn't stated, I'd
assume that they are version 1.
Personally, in your situation, my next option would be to consider an add-on
card with high speed USB ports on it... such as...
http://www.novatech.co.uk/novatech/specpage.html?F5U219
(I don't know where you are in the world.. but £15 for a decent brand of
card is OK in my book)
Perhaps someone else in this group can accurately help you determine what
version of ports you have on your Dell.
______________________________________________________
The Grim Reaper