Depends on how much money you want to spend.
I first switched to 20" rims on 255/45/20 tires.
Total cost about $2500
This helped a lot in handling and ride and body lean.
The 255/45/20's are a little beefier than 245/45/20's, and probably flex and lean even more than 245/45/20's.
This site will help you figure out which tires are closest to stock.
Car / SUV / Van / Light Truck Tire Calculations
I then put on Hotchkis sway bars about 2 weeks later but didn't really notice that much improvement in handling/leaning.
Total cost $340 for the bars, plus $265 labor to install.
Then I added KWv2's and the ride became perfect. The KW's lowered the car a little over an inch, which really helps reduce body lean.
Total cost $1,420 for the coilovers, plus $400 for installation and $110 alignment.