i am looking to upgrade my brakes but don't have the $1000's of dollars for big brake upgrade. i already have a set of 4 brembo slotted/cross-drilled rotors (oem size... front and rear). is there a caliper upgrade for use with oem size rotors available?

Im pretty sure the GSX brake upgrade fits oem size calipers. that and its not expensive at all $100-200 campared to $1000. theres a pretty big difference in the breaking as well compared to stock.

http://forums.2gnt.com/dcboard.php?az=show_topic&forum=6&topic_id=29580&mesg_id=29580&page=

http://forums.2gnt.com/dcboard.php?az=show_topic&forum=6&topic_id=17453&mesg_id=17453&page=

Here. These are found at the very top of this forum. Try to utilize the search in the future homie. ;)

so if i was going to do the GSX upgrade I'd have to get the GSX rotors correct and not use the current brembos i have?

correct
you can still use the Brembo rear ones though

i do caution the use of the brembos though, if they are cross drilled and sloted, and you got them off ebay they are NOT authentic.
the seller uses Brembo blanks and then cross drills them them selves.
these are then prone to cracking under stress of "spirited" driving.

Brembo does make either drilled or slotted rotors (www.machv.com)
and will back them up. ($160 for the fronts, $250 for the rears)

if this is a track bound car i would go with the slotted rotors and EBC pads for Awesome braking.
SS lines and new clean brake fluid will help the braking feel as well and are worth the additional upgrade

i got my brembo's cross drilled/slotted for 105 new on ebay and came in sealed brembo boxes

Brembo themselves DO NOT make a cross drilled and sloted rotor, its either one or the other.

take a look at the rim of the rotor
there should be stamping marks with a Serial # and a min thickness stamp as well as the Brembo logo
if its not there they are Fakes
if it is there take the S# and call brembo and they will tell you that it was a standard blank

plus all Brembo rotors are Cadmiun plated and will be a yellow gold color