Hey
From the standard RB30ET wit just the usual cam, chip, exhaust mods u can get bout 160-170rwkw.
U can get a Turbo from ATS for bout $1100 exchange that wil get u very close to your 500hp but will prob need stronger pistons altho the RAJAB boys used standard pistons in there 9second calais but u will no doubt need a computer upgrade.
But then when u go for the V8 your lookin at more rego,most likely more fuel,more money to get it to start wit,n most likely more hasles to convert it to a V8