HD555 are the minimum you should consider.
But remember that to perform optimally, you need an amplifier or amplified sound card.
Such as the Asus Xonar Essence ST/STX (most popular):
±R1250 Asus Xonar Essence STX , ultra fidelity 7.1 pci-express sound card , with metal shielding , built-in High Fidelity headphone amplifier , ASUS AV100 High-Definition sound processor , support DS3D GX 2.0 , VocalFX , Dolby®/DTS + Pro Logic IIx , SNR output/input : 124/117db , HD Audio at 192KHz/24Bit , EAX advanced HD , with optical SPDiF out + HDMi out ( via header to VGA card with HDMi out ) + RCAx2 analog out + no 3.5mm + CD-In + Aux-In - retail pack + bundled software
If you can't afford that/don't want it/etc. then look at the Grados as per the OP. He's getting them because he doesn't have an amp.