Depends on your budget and your needs. I just recently purchased a computer and monitors, the discussion regarding it is in the "Trading Computer" thread. While I am not an expert by any means, feel free to ask me any questions.
I keep track of this in MS Word to see how my systems/software change over the years. Build the towers myself. FWIW, here it is... Trading system: Intel 2600K - Asus P8 68V-GEN3 - 16GB RAM Corsair- Intel 320 SSD OS / WD Raptor Storage - (2) x PNY Quadro NVS 300 - Corsair HX750 - Antec P180B - (4) x 24" Samsung PLS 850 Series 1920 x 1200 - 19" Samsung 720p TV (Bloomberg) Software: Win 7 Pro 64 - IB TWS - CQG Integrated Client - Multicharts - IQFeed - Investor RT PRO - QCollector - Excel - .... Research - Strategy Development system: Intel 3930k - Asus P9X79 WS - 32GB RAM Corsair - Intel 520 SSD OS / (2) WD 1TB Storage- (1) PNY Quadro NVS 300 - Corsair HX750 - Antec P180B * IOGear KVM switches that allows me to use 1 of the 24" LCDs from above array. Software: Win 7 Pro 64, Multicharts, Matlab R2012b, IPython/SciPy/Numpy/Pandas, Amibroker, IRT Pro, custom utilities. Travel/Vacation Setup: Lenovo T530 ThinkPad - Intel i5 3320M - 8GB Hynix - Samsung SSD SATA3 - 15.6" 1920 x 1080 - Win7 Pro 64 (2) ThinkVision LT1421 14" Portable USB LCDs (allows for 4652 total resolution width). All components packed within a medium sized Briggs&Riley business brief for hassle-free portability. Software: Same as Main Trading