A lot of socket-754 motherboards are limited to three DIMM "sides"--that is, they can handle two or three DIMMs as long as they are single-sided. Two or more double-sided DIMMs means four or more sides, and some socket-754 boards can only handle all four by dropping to PC2700. I don't know if the TForce 6100 has this issue, but I'd check it out, first. Even if you can run 2x512MB at PC3200, though, it's still a better idea to get 1x1GB.
The difference between the GeForce 6100-M7 and the TForce 6100-754 is not entirely known, but they seem to have the same northbridge and the same overclocking features.
As for memory, the cheapest Crucial Ballistix 1GB kit (2x512MB) runs $120. That's *way* too much, considering you can get 1GB (1x1GB) for $76. Save the $44. RAM does not affect CPU overclocking. If you want to overclock the RAM, it won't get you much of a real-world performance boost. It's just a bad idea on a budget build.
As for PSUs, wattage means little. The Fortron AX450-PN is $57 shipped. On a budget build, you could do better. At the very least, go for the 400W model. It's got the same +12v rail amperages, the same efficiency and the same over-voltage protection--and it's $13 cheaper. You could also go for a 350W or 300W, which would save you even more.
You don't need a cooling fan for your RAM unless you plan to overclock it, which on a budget build is a bad idea.
Hitachi has questionable reliability. If you want to risk losing all your data for no apparent reason one day, go for it! Anyway, Western Digital offers the best price/performance ratio among major brands, not Hitachi.
If you go with these suggestions--generic RAM, a lower-wattage PSU, no extra fans and a GeForce 6100-M7--you can squeeze another gig of RAM into the equation.
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