I'd definitely keep the card (provided it's got all the I/O you want). RME do the computer interface side of things really well with low latency, good drivers, and nice monitoring mixer software, Totalmix.
Do you just want a couple of pres, rather than a mixer? The best way to spend $400 is on good quality DIY kits from FiveFish or Seventh Circle Audio. You can start an SCA rack with a couple of T15's for slightly over budget at $500. Might be one to consider if you plan on adding more pres with different flavours later.
Otherwise, you can get a two-channel, clean 'n clear SC1 from FiveFish fro $330-390.
This would give you quite a good quality set up, much better than anything you could buy for $400.
PS: if you hang on to the RME card maybe you could submit some test files to the converter comparison thread