I would agree with Brodie generally, but recommend something slightly differently.
I was going to find an example on the Dell website but they don't seem to offer (at least where I was looking) the hardware I was thinking of. I've been researching a similar rig for myself. Here's what I would look for and why:
CPU: i5-760 (~$200) or i7-870 (~$280)
It would generally be hard to notice a difference between the two except in programs which use as many cores as possible (sketchup only uses 1, though, so for just that save your money and get the 760). If the $80 isn't a big deal, the 870 is a very nice processor (for renderings, especially). Having said that, Q1 next year intel is releasing a new line of processors (the 2500k is the processor I'm thinking of) that should be about (from early testing) 20-25% faster for (speculation) about the same price. It's not a long wait and might be worth it.
Memory: If you want to use a lot of textures and/or if you would want to do renderings also, more matters. If it does, 8gb would be nice and 4 otherwise. The processors I've listed are "dual-channel" and work best with 2, 4, or 8gb. If you got an i7-9xx (triple-channel) make sure you get 3, 6, or 12gb.
Graphics card: If you are really only using it for sketchup a 310 is probably just fine. Certain types of rendering (the future of rendering, I was say) need a powerful graphics card and the 310 might be a bottle-neck. A GTX 460 1gb (~$220) is probably the current best performance for the price. A pair of these are VERY powerful -- just make sure the motherboard supports SLI. Also, if you (read: your son) would ever play games this would be much better.
A quick search found basically the above -- of course, you would have to assemble it yourself. Also, the 2 video cards would be chopped to 1 and I'd upgrade that 1 to the 1gb variety. The links appear to be out-of-date but prices be about that.
**Processor - Intel i5 760 $209
Motherboard - BIOSTAR T5 XE CFX-SLI (don't laugh, read the reviews first) $134.99
http://www.newegg.com/Product/Prod [...] 6813138162
Memory - OCZ 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) XMP-Ready Rev.2 $94.99 (Before $20 mail in rebate)
http://www.newegg.com/Product/Prod [...] 6820227503
Graphics - 2 X Galaxy 60XMH6HS3HMW GeForce GTX 460 (Fermi) GC 768MB $359.98 (before $10 mail-in rebate per card)
http://www.newegg.com/Product/Prod [...] 6814162058
HDD + Power Supply : Samsung Spinpoint F3 + OCZ Fatal1ty OCZ550FTY 550W $99.98 (before $20 mail-in rebate)
http://www.newegg.com/Product/Comb [...] mbo.466067
Case - Antec 300 $59.95
CPU Cooler - ARCTIC COOLING Freezer 7 Pro Rev.2 92mm Fluid Dynamic CPU Cooler $35.98
http://www.newegg.com/Product/Prod [...] 6835186134
Optical Drive - Sony Optiarc CD/ DVD Burner $17
http://www.newegg.com/Product/Prod [...] 6827118039
Total (Before Shipping/Rebates) - $1011.87
After Rebates - $951.87
After Shipping - $998.87
GRAND TOTAL (After factoring mail-in rebates & shipping) - $998.87**
