recently bought a 2004 Volvo s40 1.9L T. Also new into the car scene. I want to put a cold air intake in it but I don't know if it would go where the filter goes or the turbo. If you know anything about this let me know.
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my two cents is save your money. The 1.9T doesn't draw that much air and the factory air box is well designed. Just keep the filter nice and clean. You won't get a power bump you'll probably get more noise and you'll lose the benefit of a preheat if you are driving somewhere really cold.

unfortunately, the 1.9Ts don't have a lot of after market engine parts and given your sig says you are in Cali, you may need to read up on emission rules for a 2004. If I were to invest in a gen 1, start by shopping for an ECU tune (may be hard to find) noting that may not be "legal". You can also go for a bigger intercooler, replace the 12T turbo with a 14T (some S60s used this size), shop for a 3 inch downpipe and if you like noise a free flow exhaust or muffler delete.

Alternatively, focus on the suspension and sound system. You can do struts/springs, look for a rear sway bar, 16 or 17 inch wheels, brake kits (rears are a weak spot for the Gen 1s IMHO). Sound system should be easy to do given the car uses the Double DIN size head so you have some nice options to add stuff. (something the Gen 2 S40s messed up LOL)

 

mt6127 is right - don't spend the money. In reality, independent testing finds that most "cold air" intakes reduce your horsepower and mileage, because they draw air from the hot, nasty engine compartment, instead of cool, clean air from outside (which is what the nice engineers at Volvo arranged with the superior air intake system that's currently living under your hood). The claims CAI makers make border on criminal (not sure if it's the near or far border), but it's inconceivable that they'd do any good unless they incorporate a fresh air "ram" intake system.