Definitely the 2nd computer for the processor wise. For your second post, Intel currently stand as best for processors due to their release of Core 2 duo, but out of a pentium D 805 and athlon 64 X2 4200 the athlon would be better. I would fork the extra £26 for the athlon but one thing that concerns me is the graphics. I wouldn't have a powerful processor and a good amount of ram with a 128mb NVIDIA 6150 graphics.
To me the two important things to have when buying a new PC are balance and upgradability.
Regarding balance, there is no point having a PC with a mega graphics card for example if the processor drags it down. Look for a balanced set of components. Regarding upgradability, make sure whether it has onboard graphics or not, that it has a PCI Express slot so you can install a new graphics card. Also make sure that there is a slot free for adding more memory if you want. Other things to look for are that the PC has SATA for the hard drives or better still SATA II which is faster.
Regarding the two PCs I personally like the second one, the AMD based one better but would consider another company and a different PC. A bigger hard drive would be nice as would a proper graphics card. Must admit its cracking value though at that price.
Agree with SG Atlantis® that on the face of it the second pc (Athlon 4200 X2) appears to win hand down.
But (there's always one of those!):
- 250W PSU is a bit weak should you later want to install a more powerful graphics card - anthing above a NVidia 7600GT (which is in itself a pretty decent card) is likely to need additional power. Personally I would upgrade to a 550W PSU - a £25 option on the Mesh website
- Only one optical drive. Not necessarily a problem to most users.
- Note the lack of speakers. Mesh site has options available from £10 (ugh!) upwards. Personally I'd go for the iTrigue 2.1 speakers at £45
But as DieSse said, it all depends on what you want to use the PC for. If you aren't into games, then its a very decent PC as is (with speakers of course)