The bull run in Rambus ( RMBS) got fresh fuel Tuesday when the chipmaker bumped up revenue guidance. In a two-sentence news release, Rambus said it was revising its first-quarter revenue guidance to $45 million to $48 million, up from $41 million to $43 million. The company's first quarter ends March 31. One analysts surveyed by Thomson First Call was forecasting sales of $42.6 million for Rambus' first quarter. The company didn't say anything about earnings; the single-analyst estimate is for income of 6 cents a share. Rambus' shares have more than doubled already this year as optimism has grown about its prospects in a series of intellectual-property suits. After closing at $34.40 Monday, the shares were recently up $1.25, or 3.6%, to $35.65.