The chips generally benefit ATX cars more because of improved shift points.
IMHO, the first mods you should do are the intake (KKM or similar) and exhaust (SVT dual setup or other custom).
I think many would agree with this.
As far as handling goes, at a basic level tires make a big difference, look to upgrade your tires (and wheels if you can afford it) then look to springs and struts (this is where more money comes in).
