PC won't boot with more than four of same graphics card installed

[[XC] Oj101]

Ok, here's the deal. The specs of the system are as follows:

Asus P6T7 WS
Core i7 920
6x1GB Kingston DDR1866
5x Asus Radeon HD5450
2x Asus Radeon HD4650
2x500GB Seagate Hard Drives
1x 1TB Seagate Hard Drive
Windows 7 Ultimate 64bit
CoolerMaster 600w PSU

I've tried multiple driver versions from about 10.2 to current and the issue is always the same. I've also tried a different PSU and different RAM. No matter what the configuration, the system is PERFECTLY happy with four or less HD5450s installed, as soon as a fifth is added either with or without any of the HD4650s it blue screens on startup. Nothing is overclocked, the RAM is at 1600 9-9-9. I know this is a driver issue of sorts. The system also works fine with all seven cards with the "Standard VGA" driver. The blue screen is as follows:

SYSTEM_SERVICE_EXCEPTION

*** STOP: 0x0000003B (0x00000000C0000005, 0xFFFFF88003D5A080, 0xFFFFF8800586BB70, 0x0000000000000000)

*** atikmpag.sys - Address FFFFF88003D5A080 base at FFFFF88003D40000, DateStamp 4c75178

The underlined addresses vary, the addresses in red are always linked minus the preceding 0x. As soon as one of the five cards is removed, the machine boots up perfectly. Please help

Just as an example, this is what I got the next time I booted:

SYSTEM_SERVICE_EXCEPTION

*** STOP: 0x0000003B (0x00000000C0000005, 0xFFFFF880041C0080, 0xFFFFF88006869B70, 0x0000000000000000)

*** atikmpag.sys - Address FFFFF880041C0080 base at FFFFF880031A6000, DateStamp 4c75178