As many have said, a PC is both cheaper and more powerful for CS5.5 than a Mac. I too am a Mac user - 12 years, FCP 1.25 - FCS3. I feel yourt pain, but a PC is the better solution. Here is a system I have suggested more than a few times this week:
Here is your parts list from Newegg. You will toss another $100 or so in cables and fans, etc.
Motherboard: ASUS P8P67 DELUXE (REV 3.0) LGA 1155 Intel P67 SATA 6Gb/s USB 3.0 ATX Intel Motherboard with UEFI BIOS: $209.00
Processor:Intel Core i7-2600K Sandy Bridge 3.4GHz (3.8GHz Turbo Boost) LGA 1155 95W Quad-Core Desktop Processor Intel HD Graphics 3000 BX80623I72600K: $320.00
RAM:G.SKILL Ripjaws X Series 16GB (4 x 4GB) 240-Pin DDR3 SDRAM DDR3 2133 (PC3 17000) Desktop Memory Model F3-17000CL11Q-16GBXL:$150.00
NVidia GTX570: EVGA 012-P3-1571-KR GeForce GTX 570 HD w/Display-Port (Fermi) 1280MB 320-bit GDDR5 PCI Express 2.0 x16 HDCP Ready SLI Support Video Card: $340.00
HARD DRIVES (5): HITACHI Deskstar 7K1000.D HDS721010DLE630 (0F13180) 1TB 7200 RPM 32MB Cache SATA 6.0Gb/s 3.5" Internal Hard Drive -Bare Drive. $130 EACH OR $650.00
CASE: COOLER MASTER HAF 932: $140.00
POWER SUPPLY: Antec EarthWatts Series EA-750 Green 750W ATX12V v2.3 SLI Certified CrossFire Certified 80 PLUS BRONZE Certified Active PFC Continuous Power Supply: $100.00
BLURAY BURNER: LG Black 12X BD-R 2X BD-RE 16X DVD+R 12X DVD-RAM 10X BD-ROM 4MB Cache SATA Internal 12X Super Multi Blue with 3D Playback Blu-ray Disc Rewriter WH12LS38 LightScribe Support - OEM: $70.00
I trust this helps.