That depends on what you prefer (ATI, nVidia) and wat you want to do (gaming, working). Another issues are the price and at least the power consumption. You can buy a monster graphics card, but f.e. a Radeon HD4870 needs up to 285W power. I compared a long time and came to the ATI Radeon HD2600XT. It's fully supported, Dual-Monitor, TV-Out, and needs only 50W power. There are also various heatpipe versions out there, so no noisy fan. The version from Gigabyte is only 1 slot wide, great for microATX boards. Costs on ebay are between 30 and 50 Euro. If you want to check out other ATI Radeon HD series cards, take a look at the drivers list.

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2 Opteron systems: OSx86 10.5.8, Andy's 9.8.0 kernel, Asus A8N-SLI Premium, Opteron 185 o'clocked @ 2 x 2,95 GHz (2nd system 2.6 GHz), ATI Radeon HD2600XT 256MB Dual-Monitor 2x HP L2035, 4 GB RAM, Griffin FireWave as main audio device, Marvell + nForce LAN, Asus U3S6 USB3/SATA6 card, 5,5 TB harddisk, Firewire 800 card, Apple Remote + eHome IR receiver, 2x Wacom serial graphics tablet, Canon Pixma iP4700, Logitech Internet Navigator wireless keyboard/mouse combination.

My Audio stuff: M-Audio Transit USB (default audio), M-Audio ProFire 610, M-Audio ProFire Lightbridge (34 channels) using Creamware A16 ADAT converter • MIDI: M-Audio Midiman 4x MIDI interface • Behringer Audio Mixers: Xenyx 1002, Xenyx 1002FX, Xenyx 1202FX, Eurorack UB1002FX, Eurorack MX1804FX, Eurorack MX262A • FX devices: Lexicon MPX100 DSP, Behringer DSP-1000 Virtualizer, Behringer MiniFEX 800 DSP, Behringer Multicom Pro MDX4400 compressor • RETRO: MSSIAH midi/sequencer/synthesizer cardridge for the C64 (Dual-SID), Steinberg M.S.I. MIDI Interface for C64