I'm in a similar situation to you, although I'm not really a gamer. I am leaning towards a Soltek MoBo though....
I have conceptually built these systems using prices from NewEgg as of 2/16:
Note: The case and LCD I have already purchased, so those are the prices I paid, not NewEgg prices.
Socket 754 System
SOLTEK "SL-K8TPro-754" VIA K8T800 $106.00
AMD Athlon 64 3000+, 512KB L2 Cache 64-bit Processor $147.00
Kingston 184 Pin 512MB DDR PC-3200 - Retail $69.58
SAPPHIRE ATI RADEON 9600PRO Advantage Video Card, 128MB DDR, 128-Bit, DVI/TV-Out, 8X AGP, Model "100561 GR" -BULK -OEM $103.99
Seagate 80 GB Barracuda 7200RPM SATA with NCQ Hard Drive, Model ST380817AS, OEM $73.69
NEC 16X Double Layer DVD±RW Drive, Black w/ Software, Model ND-3520A, OEM $66.49
Microsoft Windows XP HOME Edition With Service Pack 2 -OEM Specifications: Designed exclusivel...... $93.94
ImageQuest L90D+ 19" LCD Monitor w/ Speakers, 8ms and DVI ?RETAIL $388.00
Antec Solution Series Quiet Black Mid Tower Case with 350W Power Supply, Model "SLK2650-BQE" - Retail $45.50
$1,094.19
Socket 939 System
SOLTEK "SL-K8TPro-939" VIA K8T800 $113.99
AMD Athlon 64 3000+, 512KB L2 Cache, Socket 939 64-bit Processor - Retail $158.00
Kingston 184 Pin 512MB DDR PC-3200 - Retail $69.58
SAPPHIRE ATI RADEON 9600PRO Advantage Video Card, 128MB DDR, 128-Bit, DVI/TV-Out, 8X AGP, Model "100561 GR" -BULK -OEM $103.99
Seagate 80 GB Barracuda 7200RPM SATA with NCQ Hard Drive, Model ST380817AS, OEM $73.69
NEC 16X Double Layer DVD±RW Drive, Black w/ Software, Model ND-3520A, OEM $66.49
Microsoft Windows XP HOME Edition With Service Pack 2 -OEM Specifications: Designed exclusivel...... $93.94
ImageQuest L90D+ 19" LCD Monitor w/ Speakers, 8ms and DVI ?RETAIL $388.00
Antec Solution Series Quiet Black Mid Tower Case with 350W Power Supply, Model "SLK2650-BQE" - Retail $45.50
$1,113.18
Socket 939 System PCI-E
SOLTEK "SL-K890Pro-939" VIA K8T890 $127.00
AMD Athlon 64 3000+, 512KB L2 Cache, Socket 939 64-bit Processor - Retail $158.00
Kingston 184 Pin 512MB DDR PC-3200 - Retail $69.58
Albatron nVIDIA GeForce 6600 Video Card, 128MB DDR, 128-Bit, DVI/TV-Out, PCI-Express, Model "PC6600/128" - Retail $119.99
Seagate 80 GB Barracuda 7200RPM SATA with NCQ Hard Drive, Model ST380817AS, OEM $73.69
NEC 16X Double Layer DVD±RW Drive, Black w/ Software, Model ND-3520A, OEM $66.49
Microsoft Windows XP HOME Edition With Service Pack 2 -OEM Specifications: Designed exclusivel...... $93.94
ImageQuest L90D+ 19" LCD Monitor w/ Speakers, 8ms and DVI ?RETAIL $388.00
Antec Solution Series Quiet Black Mid Tower Case with 350W Power Supply, Model "SLK2650-BQE" - Retail $45.50
$1,142.19
As you can see, to go from socket 754 to socket 939 cost an additional $18.99. To go from AGP to PCI-E on socket 939 costs an additional $29.01. Total cost to go from 754 AGP to 939 PCI-E is $48.
For my money, the $48 is definitely worth the future upgradabilty. I'm going 939 and PCI-E.
Best of luck.