For a Nikon D90, card speed matters most if you shoot bursts or bracketed sequences, but you don’t necessarily need the fastest card available. A Class 4 or Class 6 card can work fine for general shooting, and the camera body itself may become the limiting factor before very fast cards make a big difference.

A few things to consider:

• Write speed, not just read speed: manufacturers sometimes advertise high read speeds, but write speed is what affects in-camera performance.
• Reliability/brand: stick with reputable brands such as SanDisk, Lexar, or Kingston.
• Capacity: choose based on your shooting style and workflow. Around 8GB to 16GB is a reasonable range for general use; if you shoot a lot of video or want enough space for a full day, go larger.

In short: prioritize a reliable card from a good brand with decent write speed. For most D90 users, a midrange card in the 8GB–16GB range is a practical starting point.