It's OK to use different brands of RAM as long as all of the ram is guaranteed compatible with your model Mac. Each pair of RAM modules should be the same brand and configuration within the pair, however.
The TigerDirect RAM makes no mention of Mac compatibility, so you are taking a chance. The $40 advertised price is after two mail-in rebates, but they don't display the rebate terms and conditions, you have to go to another website and look it up. Commonly, there is a limit of one rebate per person. Since you have to buy a pair of DIMMs for the G5, you could pay $40 for the first one and $90 for the second. Also, Ultra rebrands other people's memory, so you have to be careful to see you get a matched pair... there's no telling whose chips it will be from week to week. Last point: You have to pay tax on the full $90 price, you don't get the tax back with the rebate, so it is more like $47 rather than $40.
My stock advice: Buy Mac RAM from a seller who gurarantees it is compatible with your Mac.
