I use dd-wrt .23 without problems.
About your prefs:
Your download-limit ought to be 500kbyte/s (4000kbit/s / 8), no reason to limit this.
You should increase your upload to about 32kbyte/s to get more credits (and as a result a higher possibility of a download)
The max. connections are way to low, set 'em to about 400, unless you like the flat line in statistic and the problems with download.
I'ld recommend to set the max new cons / 5sec to about 25.
The upload queue should be increased as well (~3000 should be OK), it won't kill your sys, but you may 'loose' valuable sources, cause they uploaded to you, the got credits, but as long as you don't upload to 'em, it's not very possible to get more chunks by 'em.