The 300 is a good choice, plenty of room for the 4870 and the rest of those parts.
The PSUs you've listed so far have just a single PCI-e power connector for the video card and 4870 wants 2. You could cobble together the 2nd power lead with the molex to PCI-e adapters that come in the accessory pack of most 4870s. Or you can go with a PSU that has 2 PCI-e connectors for a cleaner look inside the case. OCZ ModXStream Pro 500W PSU $40 after rebate Be sure to check the shipping cost on the PSU to your location. You can sometimes a PSU with free shipping that works out to be a better deal.
Do not get either of those Thermaltake PSU's. They're both low quality PSU's with no active PFC, low efficiency, and not enough power output or connections to properly power a 4870. +1 to the OCZ ModXStream 500W PSU WR2 linked above. You do NOT want to try and save a buck on the PSU.