I compared the 720x3 vs the lower cache, but quad 620x4 here:
http://www.anandtech.com/bench/def [...] =106&p2=83
The 620 would better suit your needs and with a mild OC, you'll actually do quite well with it over an x3. It's also cheaper.
As for the OS, its hit or miss whether it will work or not. Depends on how gateway slipstreamed the OS into the startup. Legally, it's shouldn't be used.
So here's a shot at extreme budget: (w/fs = free shipping)
AMD 620x4 $99 w/fs
http://www.newegg.com/Product/Prod [...] 6819103706
Asrock 785G AM3 mobo $85 w/fs -Can't CF, but is GPU expandable, and very good IGP, see here:
http://www.tomshardware.com/review [...] ,2381.html
http://www.newegg.com/Product/Prod [...] 6813157168
If you want the CF as an option, get this +$15
http://www.newegg.com/Product/Prod [...] 6813128398
Gskill 1333 4gb cas 9 $71 w/fs (not worth the extra to get 1600 or heatshields for your needs)
http://www.newegg.com/Product/Prod [...] 6820231253
OCZ StealthXstream 400w PSU $40 w/fs and $10 mir (Enough to add a 4670 or similar caliber GPU. If you want a bigger GPU option, then look in the 600w range, it will add $25-$30)
http://www.newegg.com/Product/Prod [...] 6817341020
Case: Cases are personal choices, they will all do the same basic stuff. Go for free shipping as it will save $20 and that's super important at this price point. I budgeted $50 to give you a few choices, spend more, spend less, just adjust my final numbers from $50 w/fs.
http://www.newegg.com/Product/Prod [...] rder=PRICE
Spinpoint F3 500gb $50 w/fs Best bang for buck HDD. If you need more capacity, the 1tb drive is $85
http://www.newegg.com/Product/Prod [...] 6822152181
Dvd Burner $31 Very high rated unit
http://www.newegg.com/Product/Prod [...] 6827151187
If you do find out you need an OS, buy this combo with the CPU. Win Vista Home Premium will only add $80 with the combo:
http://www.newegg.com/Product/Comb [...] 19-103-706
Total w/ OS: $514 - $10 mir = $504
Total w/o OS: $434