ati hd 2600xt or 8600 GTS for +$100?

not 100 bucks extra worth... you should just build your own system, or contact me to do it for you if you don't feel you can do it. I run a sort of small business doing this type of thing out of my home, mostly local. We could talk and find out what you want and I bet you my price will be a lot lower than Dell... and i'll send you all the manufacturer paperwork too so you'll have the parts covered by the mfg. or you could just take the cheapest option, and then spend 160 bucks and buy an ati 3870 and install it yourself. have fun either way.

No. XT is a good card.
Get the cheapest card offered when configuring your system and buy the card you want at a cheaper price (if you decide to upgrade at a later date).
Use the $ you save and get the 425w psu instead of the 375w for future upgrades.
The 375w is an excellent psu @ 30amps but why not?

The HD2600XT will often beat a 7800GTX though. Oblivion, Crysis, COD4, UT3, Orange Box, to name some games.

OP: Yeah, no way is the 8600GTS worth $100 over the HD2600XT. It is generally better, so $20-30 would be reasonable. But like Cleeve said, beyond that you want the HD3850.

Get the cheapest card possible and upgrade with a card from Newegg like the 38xx.

2600 comes in a variety of flavors and i doubt they're going to tell you which one they're giving you. There is one with 256megs of gddr3, one with 512 of gddr3 and one with 256 of gddr4. the performance differences between them should be taken into consideration if you want to get that card and keep it... if you want to keep that one, find some charts for that particular card. i think the 'xt' covers the 256 gddr3 the 512gddr3 and the 256 gddr4. up to you on what you decide. I did see a chart however that i think covered the 512gddr3 and it didn't do bad in crysis, keeping ahead of the 8600gts i believe at medum settings.