Dad's Budget PC

My father's old Win98 machine is finally due for an upgrade (according to my mother), and I've been asked to put together a budget build. I've perused the stores a bit, but I'm not a true expert, so I'm asking for advice.

APPROXIMATE PURCHASE DATE: Within 2 weeks
BUDGET RANGE: $300 variable

SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Web surfing/email, Flight Simulators (Dad's retired Air Force!)

PARTS NOT REQUIRED: keyboard, mouse, monitor, speakers, OS (will use XP or Windows 7, whichever is best for the parts I get. I also have a mid-size case that I can use (not sure of brand/model, but it's fairly new)

PREFERRED WEBSITE(S) FOR PARTS: newegg.com, also use tigerdirect and microcenter (got a microcenter store local, too)

PARTS PREFERENCES: None, but betting on an AMD processor to get more bang for my buck

OVERCLOCKING: No
SLI OR CROSSFIRE: No

MONITOR RESOLUTION: 1024x768 - 1680x1050

ADDITIONAL COMMENTS: reliability is the key - Dad keeps these things well past the date we'd all prefer to upgrade.