If it was be on this decision I focus on best single core performance when I plan builds for friends, that way you get the longest terms of compatibility for your system when it comes to focusing on just memory and graphics upgrades.
To be honest, start small but plan big.
Get the beefiest CPU you could get your hands on with a feature-filled ATX motherboard, preferably Intel since they offer SLI/Crossfire cross-compatibility. If your looking for a model recommendation it'll depend on how you want to use your PC feature-wise. I go for any motherboard that'll support Multi-GPU for the simple fact for performance per dollar.
As for your memory, for budget purposes, start at 4GB or 8GB if you are a power user since that seems to be enough for today's standards.
For video cards, grab an HD 7850 if you want to have a kick start on game compatibility, the 1GB if you use low-res (1600x900 or lower) or 2GB if you are a 1080p user. IF you feel the 7850's price is too much go after a feature-filled 7770 instead.
As for your case, go after something traditional but open ended, such as Antec. The Three Hundred cases have always been my favorite pitstop for a decent quality case under the $100 range. IF you feel the price is much look into Cooler Master if you feel to compromise. Keep in mind however that picking a quality case is extremely important if you want your system to last a long time.
Power supplies. Go after Corsair's CX series since for the cost they produce little jitter therefore producing very clean outputs for your PC. This is one of the components that get the most overlooked by OEM brands and is the component that fails the most in a PC. Rosewill is another choice if your seeking a good way to save some dollars. Never grab a generic PSU, my friend learned the hard way by having to replace his motherboard and CPU.
As for hard drives, their performance seems similar enough but the best value starters are Seagates 'cuda 12 series or Western Digital's Black series. To get the best bang for your buck, go 1TB as there's often a price jump after that. You can also grab a second hand hard drive as long as the diagnostics on them are green. SSDs are still changing rapidly so they aren't the way to go just yet.
As for your budget recommendations, if you seek a PC that'll last you 3 years easy the budget should be 700 in balance to today's games. If you wanna save money on the long run for a PC that's got 5 years of capable mileage itll be 1000 or over.
