If by upgrade you mean you want more power take a look at burger tuning. most people run some sort of variation with a juicebox. I ran a JB+ on mine (easy to remove and install takes less then 5 minutes plug and play) and gives 40HP to the wheels at stock setting.
They have further tuning chips as well but i liked my JB+ i ran for about 3 years until i sold the car. I have a jB+ for sale currently for 200 shipped if your interested let me know. I just sold my 335i.
Do a quick search on JB+ on these forums youll see tons of 335i owners have some sort of juicebox on thier car.
Currently Drive: 2011 S4 Prestige, Quartz Gray, Black Alcantara, Adv key, B&O, NAV + rear sensors, 19's, Carbon Fiber Trim, iPod...
Previously drove:2007 335i Coupe, Titanium Silver Metallic,Black Leather, Gray Poplar trim, Premium package, Cold weather package, Sports package, i-Drive, Rear sensors, iPod.
Previously drove: 1998 3000GT Vr-4 Red/Black TT AWD AWS