UPDATE 1-TSMC Q2 sales beat f'cast, more gains ahead

* Books largest monthly sales in eight months

* Q2 sales surpass TSMC own forecast

* Migration to advanced process technology seen smooth

* TSMC, UMC shares gain, outpacing big board (Recasts with quotes and details)

By Baker Li

TAIPEI, July 10 (Reuters) - Top contract chip maker TSMC (2330.TW) booked its biggest monthly sales in eight months that topped the level by cross-town rival UMC (2303.TW), thanks to growing demand for computers and other consumer gizmos.

April-June sales jumped 88 percent from the first quarter and beat TSMC's (TSM.N) own forecast made in April, boding well for the third quarter as analysts expect TSMC to boost shipments of chips made by more advanced technology to its clients.

Taiwan Semiconductor Manufacturing Co Ltd (TSMC), which grabs about a 50 percent share of the $20 billion market for custom-built chips, had unconsolidated sales of T$25.78 billion ($781 million) last month, the company said on Friday.

The result fell 9.6 percent from a year ago but was up about 5 percent from May, marking its fourth straight monthly rise.