Make sure refurbs are a better value than new PCs of similar specs, else you'll be missing the point of getting refurbs in the first place.
As far as Dell refurbs vs new PCs, both are OK since the former's warranty will usually be on par with the latter's warranty. Any problems, and they're obligated to take care of it. Make sure other warranty is included if you want it. Stuff like on-site service is included with new units for only a year, but they may not include that with refurbs.
Another note: If u read the sales contract for new PCs, much of it may end up being refurbed parts, as it states "if Dell can't find new parts, we can use 'like-new' parts instead". E.g. they can't find a 300GB SATA hd for your brand new PC..... someone else returned their entire PC order after 2 weeks of use. Then they would swap out the SATA hd from the returned unit and put it in what would be your "brand new" machine. Everything would be brand new except the 2 week old (or in other cases even older) hd. Some parts may have been sitting around or been used in customers' PCs for many months by then
I bought a new PC..... Dimension 8300 from Dell. First one, all USB ports didn't work. Tech came in and replaced the motherboard. Now the ethernet/internet wasn't available. 3rd time, they replaced the whole system. I was told it would be a refurb. I didn't mind despite paying for a new PC since there's a good chance this time, everything would be right. I was right. 3rd time, the refurbed worked nicely. They tell u new PCs are "rigorously quality tested" for hardware and software problems, but the new PC and replacement mobo were problematic. IMO, It wasn't unreasonable to think that if a new PC had problems, a refurb would work fine.
Conclusion, go for the refurb, esp. if u can get a good deal. Make sure any specific warranties u want (besides the standard limited like on-site, or spyware removal help) is included