First off you must have 2 ps2 memory cards. Second you should get about 100,000-2 mill.[depending on the car].

Next-load data off mem card 1. Go to car and buy it then use anymoney you have left to hike up the hp [horse power] to your desired level. Then save that memory to slot 2's memory card.

After that saves go to the main menu and go to load and load data from memory slot 1 go to your home hit trade load cars from slot 2 and scroll down to the car you had gotten really powerful and buy it for the car dealer's price

Example-

I made myself 400,000and went to he dealer ship. I bought a dodge viper[any car works I just like the viper] i bought the things in order,

1. na tune up 1
2. racing muffler
3. na tune up 2
4. port and polish
5. full engine balancing
6. racing computer chip
7. na tune up 3

Then i bought the drivetrain stuff if you don't want to u dont have to but I recomend you do buy atleast the full racing tranmission. and after i got the drive train things I bought the brakes and suspension upgrades and made it almost perfect.

After i bought all the upgrades i spent maybe 350,000 saved to my second memory card. got my $400,000 back by loading and went to my home clicked on trade loaded the cars from slot 2 and bought the viper for estimated 80,000. After i checked the horepower of my viper and it had about 1080 horse power and i took it to the oil change place and changed it. After i went back to my garage and looked it then had 1110 horse power.

Well hope it works for you like it did for me, and if u cant get the na tune-up replace those with the turbo kits in the exact oder 1,2,3,and 4 if possible. Good Luck!