Based on input received so far, here's where I'm at in terms of possibilities/prices. Due diligence done via pricing at Newegg, benchmarks at Tom's Hardware. (Of course, once I settle in on "what" I'll shop the prices more extensively, just doing Newegg now for "apples to apples" value).
I was *really* wanting to spend in the $600-$700 range to get my desired performance level in AH2, and it appears I can....but it looks like there's a lot of value in terms of system longevity and ability to upgrade by going more like $1100.
$900 is significantly better than the base system, but is still Socket A, so I'm ruling that intermediate step out - gonna either go cheapo or an $1100 AMD 64 with an SLI board so I have as much upgrade headroom as possible. (I could go $1300 and run the SLI w/2 6600GTs, but don't think I need it now and can always add the 2nd card whenever)
MUCHO thanks for all the input so far, specs/prices appended, comments welcome

culero
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Socket A
Athlon XP 2700+2.17GHz, 333MHz FSB, 256K $109
Soltek SL-75FRN2-RL nForce2 Ultra 400 $65
Corsair Value Select 184 Pin 512MB DDR PC-3200 $126
RADEON 9600PRO Video Card, 256MB DDR (Sapphire) $111
Antec P4 ATX12V 450 Watt P.S. $76
RAIDMAX Beige 10-bay Case, Model ATX-208 $19
Maxtor Ultra Series Kit 80GB 7200RPM SATA Hard Drive $91
NEC 1.44MB Model FD1231H $8
ASUS 52X32X52 IDE Internal CD-RW $28
total $633
upgrades
Seagate 160GB Barracuda 7200.7 7200RPM SATA Hard Drive $34
nVidia GeForce 6600 256MB DDR (Aopen) $45
ENERMAX CSX656TA-MW case $39
Socket A (2)
Athlon 3200+ Barton 2.2GHz, 400MHz FSB, 512K $155
Soltek SL-75FRN2-RL nForce2 Ultra 400 $65
Geil Ultra dual channel PC4000 2X512 $179
nVidia GeForce 6600 256MB DDR (Aopen) $156
Antec P4 ATX12V 450 Watt P.S. $76
RAIDMAX Beige 10-bay Case, Model ATX-208 $19
WD740GD Raptor 74GB 10KRPM SATA $181
NEC 1.44MB Model FD1231H $8
ASUS 52X32X52 IDE Internal CD-RW $28
total $867
upgrades
ENERMAX CSX656TA-MW case $39
ASUS A7N8X-E Deluxe nForce2 Ultra 400 $26
Socket 939
Athlon 64 3200+ Winchester 2.2GHz 512K (25% increase) $191
GIGABYTE GA-K8NF-9 $116
Geil Ultra dual channel DDR PC4000 2X512 $179
nVIDIA GeForce 6600GT Video Card 128MB DDR3 (Aopen) $194
Antec P4 ATX12V 450 Watt P.S. $76
ENERMAX CSX656TA-MW case $58
WD740GD Raptor 74GB 10KRPM SATA $181
NEC 1.44MB Model FD1231H $8
ASUS 52X32X52 IDE Internal CD-RW $28
total $1,031
upgrades
ASUS A8N-SLI (2XPCI-Ex16, GB LAN, ALC850 8-Ch) $59
MSI K8N Neo4 Platinum SLI (2XPCI-Ex16, 2xGB LAN, SBLive7.1) $69
Antec TRUE550 550W P.S. $23
Athlon 64 FX-53 Clawhammer 1MB (50% increase) $536
(***dreaming***

)
video upgrades
nVIDIA GeForce 6800 Video Card, 256MB DDR (Leadtek) $95
nVIDIA GeForce 6800 GT Video Card, 256MB GDDR3 (eVGA) $205
ATI Radeon X800 XL Video Card, 256MB GDDR3 (MSI) $155
nVIDIA GeForce 6800 Ultra Video Card, 256MB GDDR3 (eVGA) $305
All-In-Wonder RADEON X800 XT Video Card, 256MB GDDR3 (ATI) $294
nVIDIA GeForce 6600GT Video Card 128MB DDR3 (Aopen) (2X-SLI) $194
high performance (3200+ Winchester) (6600GT X2-SLI) $1,317
max performance (FX-53 Clawhammer) (6800GT X2-SLI) $2,263 ~sigh~
best value (basic Socket 939 + SLI board + 550W P.S.) $1,113