It's not bad, though a Socket T (also known as LGA 775) mainboard will have a longer lifespan. Intel is (albeit slowly) phasing out Socket 478 for the LGA 775 socket. Celeron D CPU's come both in Socket 478 and LGA 775. The 478 mainboard will be cheaper, but eventually, Intel will phase out production of Socket 478 CPU's.
Personally, I think it's not going to matter much one way or another for you, by the time you change CPU's on this system (if it's the one I think it is) you may change mainboards too...and there will still be quite a few Socket 478 CPU's on Ebay. Most important is getting a quality mainboard for your money, whether it be Socket 478 or LGA 775.
P.S. The Celeron D chips for Socket T also feature Intel's Execute Disable bit, which are supposed to reduce the chance of buffer overflows when running Windows XP SP2. Since hackers often use buffer overflows as part of exploits to take over systems, this is considered one more line of defense. Honestly, I think having good antivirus, a firewall, and some utilities to prevent spyware are bigger deals, though it can't hurt.