Hi,
I've got some experience with large excelsheet, using the PHPExcel library. But rather than reading I used PHPExcel for writing. The biggest files I worked with had around 30 columns with 150K rows, giving less than half the number of cells you have. With these kind of numers, I was really pushin the limit of my hardware:
2x quadcore xeon + 24GB memory (memory being the main bottleneck).
As you have more than twice the number of cells, I fear you are trying to push PHPExcel a little too far, but perhaps Mark has some tips for you!
(I eventually ended up writing my own xlsx writer as datasets became to big. It employs streaming to insert data into the xlsx file, so memory is not an issue anymore)