The most important parts of your car, suspension and brakes. Power is no good if you can't put it on the ground or carry it through the corners, and racers know it's not who goes faster, but who stops quicker (i.e. who drives faster, longer).
Springs & Sway Bars
I didn't want to slam my car to the ground, but I wanted it lowered for the obvious reasons: better handling, no wheel gap, lower center of gravity. Eibach is a well known and respected name, so I had no problem when it came to deciding which springs to buy. I can't say enough good things about them, either. They really set the car down over the wheels nicely, but not too low where my shocks are no longer effective. I heard nightmare stories about really harsh rides once you lower you car, but the Eibachs are really smooth.
The combination of Eibach springs and anti-roll bars really helped my Probe hug the corners. Probes handle well even in stock form, but with stiffer springs and thicker anti-roll bars with the polyurethane bushings I can keep up with the best of them through the corners. The Eibach anti-sway bars keep the Probe nice and level through the turns, but surprisingly the biggest difference came from the end links. I was in Pep Boys one day when I noticed some polyurethane end link bushings made by Addco. I picked them up for about $20. Now this has got to be the best $20 you can spend on your suspension. They really stiffen things up, my car stays level through even the nastiest corners.
Here's a shot of one set of end link bushings.
Next up were strut tower braces. Unfortunately I only found one company that made a brace for the 1st generation Probe and they stopped making them in 1997. Not easily discouraged, I went to a local welding shop and had a guy there make front and rear braces out of some steel tubing (I think about 2" diameter). I had him weld the rear one in and the front one bolts to the shock towers just in case I need to take it off to get something out of the engine compartment. You can see both braces in the pictures below.
Look hard and you'll see the front strut tower brace... ...and the rear strut tower brace.
Strut braces are advertised to reduce body flex under hard cornering and braking, but the biggest difference I noticed was the increased stability under high speeds. No more shakes, rattles, or hums when I get moving; the car feels really solid.
Wheels & Tires
Next I had to upgrade my wheels and tires. I went with 17" TSW Hockenheim R wheels. I really like their touring look with the single nut in the middle (check out the pictures below). They're made of a relatively lightweight aluminum alloy and don't weigh that much more than the smaller stock alloy wheels. For the tires I choose Nitto NT450 and sized them (225/45/ZR17) so that even with the 17" rims (stock was 14") I would still be within 2% of the stock rolling diameter. I had to cut out the rear inner fender lip to keep the tires from rubbing under hard cornering and over big bumps (see picture below).
My stopping power comes from a set of Brembo cross-drilled rotors.
Front... ...and rear.