Upgrade around existing parts or wait to upgrade all at once?

I am currently rocking this beast:

Mobo: Gigabyte GA-K8NS
CPU: AMD 64 3400+ (Single Core)
RAM: Corsair XMS PC3200 DDR SDRAM
GPU:MSI GeForce 6800GT
PSU: Enermax 500W

  ...   ......      

A kind friend however is offering to give me the following for free:

Mobo: Biostar TF7025-M2
CPU: AMD Athlon 64 X2 3600+ Brisbane 1.9GHz 2 x 512KB L2 Cache Socket AM2 65W.
GPU: GeForce 7950 GX2

I've been reading for several days now and I think I have a pretty solid idea of what I would want to build if I had the money right now. Now that I've been offered these parts however, I am thinking I could build a system around them and use them as placeholders for their near-future replacements.

I earnestly submit, for any approval or suggestions: "Saving for a rainy day, budget PC"

APPROXIMATE PURCHASE DATE: This week
BUDGET RANGE: ~$350 before rebates
SYSTEM USAGE FROM MOST TO LEAST IMPORTANT: Gaming [WoW, League of Legends, L4D 1 / 2??, TF2, Quake3], Multi-tasking and web development/graphic design (hobby level)
PARTS NOT REQUIRED: OS, Peripherals, Monitor
PREFERRED WEBSITE(S) FOR PARTS: newegg.com
COUNTRY OF ORIGIN: USA
PARTS PREFERENCES: Price / Performance ratio!
OVERCLOCKING: Maybe
SLI OR CROSSFIRE: No
MONITOR RESOLUTION: 1440x990+
ADDITIONAL COMMENTS:

*italics are to denote equipment I already own*
CPU: AMD Athlon 64 X2 5600+ OEM $50.99
Heatsink: ARCTIC COOLING Freezer 7 Pro Rev.2 92mm $34.98
Mobo: Biostar TF7025-M2
GPU: GeForce 7950 GX2
RAM: ?
HDD: SAMSUNG Spinpoint F3 HD502HJ 500GB $54.99

Case / PSU:
Lancool PC-K62 and OCZ ModXStream Pro OCZ700MXSP 700W Combo $159.98
or
Antec 300 Illusion and Antec TruePower New TP-550 550W Combo $159.98

Personally I would prefer the K62 and the Antec PSU, where is the swap combo button?? This puts me at $249.95 before RAM and shipping. I'm ok with buying the Athlon 64 X2 for $50 because that will turn the extraneous equipment into a decent HTPC or Workstation, but I'm not sure what to do about the RAM. I'd prefer not to spend more than $50-60. Current RAM thoughts:
G.SKILL 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)
Mushkin Enhanced Silverline 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400)

Either would put me at ~$300 and within my budget. Would this be a sound decision or would I be better off just buying the case and PSU and some better DDR2?

Thanks everyone!