Your best bet at this point, for better performance, would be a mobo with DDR3 support.
I had a "portable desktop" before my current setup, too. A WidowPC Sting917 X2. As of this past Feb. I'd had it for 4 years. It was great until a solder joint came loose on the video card and the screen went all funky... I'm sure you notice with your Dell too, but PD's tend to run really hot and I think that's why it failed.
In March this year I bought a new system (all from NewEgg w/free shipping
).
http://www.newegg.com/Product/Produc...82E16820144241
http://www.newegg.com/Product/Produc...82E16814187069
http://www.newegg.com/Product/Produc...82E16811119196
http://www.newegg.com/Product/Produc...82E16813128359
http://www.newegg.com/Product/Produc...82E16819115037
http://www.newegg.com/Product/Produc...82E16822136284
http://www.newegg.com/Product/Produc...82E16817341022
http://www.newegg.com/Product/Produc...82E16827135188
Sorry for the slight hijack, but I'm very proud of/pleased with this system. It runs perfectly stable with the CPU OC'd to 4.0 GHz (444MHz x9). I can play EVE Online at full settings for 6+ hours without a problem. And it runs Halo PC pretty good, too. I have XP Pro installed.